Add notes on running unit tests.

Author: Arthur Evans

CONTRIBUTING.md

@@ -2,9 +2,9 @@
 
 There are many ways to contribute to the Polymer project! We welcome and truly appreciate contribution in all forms - issues and pull requests to the [main library](https://github.com/polymer/polymer), issues and pull requests to the [elements the Polymer team maintains](https://github.com/polymerelements), issues and pull requests to one of our many [Polymer-related tools](https://github.com/polymer), and of course we love to hear about any Polymer elements that you build to share with the community!
 
-# Logistics
+## Logistics
 
-## Communicating with the Polymer team
+### Communicating with the Polymer team
 
 Beyond Github, we try to have a variety of different lines of communication open:
 
@@ -14,7 +14,7 @@ Beyond Github, we try to have a variety of different lines of communication open
 * [Mailing list](https://groups.google.com/forum/#!forum/polymer-dev)
 * [Slack channel](https://bit.ly/polymerslack)
 
-## The Polymer Repositories
+### The Polymer Repositories
 
 Because of the component-based nature of the Polymer project, we tend to have lots of different repositories. Our main repository for the Polymer library itself is at [github.com/Polymer/polymer](https://github.com/polymer/polymer). File any issues or pull requests that have to do with the core library on that repository, and we'll take a look ASAP.
 
@@ -24,17 +24,17 @@ The GoogleWebComponents element product line is maintained by teams all across G
 
 We also track each element product line overall in "meta-repos", named as `$PRODUCTLINE-elements`. These include [paper-elements](https://github.com/polymerelements/paper-elements), [iron-elements](https://github.com/polymerelements/iron-elements), [gold-elements](https://github.com/polymerelements/gold-elements), and more. Feel free to file issues for element requests on those meta-repos, and the README in each repo tracks a roadmap for the product line.
 
-## Contributor License Agreement
+### Contributor License Agreement
 
 You might notice our friendly CLA-bot commenting on a pull request you open if you haven't yet signed our CLA. We use the same CLA for all open-source Google projects, so you only have to sign it once. Once you complete the CLA, all your pull-requests will automatically get the `cla: yes` tag.
 
 If you've already signed a CLA but are still getting bothered by the awfully insistent CLA bot, it's possible we don't have your GitHub username or you're using a different email address. Check the [information on your CLA](https://cla.developers.google.com/clas) or see this help article on [setting the email on your git commits](https://help.github.com/articles/setting-your-email-in-git/).
 
 [Complete the CLA](https://cla.developers.google.com/clas)
 
-# Contributing
+## Contributing
 
-## Filing bugs
+### Filing bugs
 
 The Polymer team heavily uses (and loves!) Github for all of our software management. We use Github issues to track all bugs and features.
 
@@ -46,7 +46,7 @@ We love examples for addressing issues - issues with a Plunkr, jsFiddle, or jsBi
 
 Occasionally we'll close issues if they appear stale or are too vague - please don't take this personally! Please feel free to re-open issues we've closed if there's something we've missed and they still need to be addressed.
 
-## Contributing Pull Requests
+### Contributing Pull Requests
 
 PR's are even better than issues. We gladly accept community pull requests. In general across the core library and all of the elements, there are a few necessary steps before we can accept a pull request:
 
@@ -55,20 +55,98 @@ PR's are even better than issues. We gladly accept community pull requests. In g
 - Fork the repo you're making the fix on to your own Github account.
 - Code!
 - Ideally, squash your commits into a single commit with a clear message of what the PR does. If it absolutely makes sense to keep multiple commits, that's OK - or perhaps consider making two separate PR's.
-- **Include tests that test the range of behavior that changes with your PR.** If you PR fixes a bug, make sure your tests capture that bug. If your PR adds new behavior, make sure that behavior is fully tested. Every PR *must* include associated tests.
+- **Include tests that test the range of behavior that changes with your PR.** If you PR fixes a bug, make sure your tests capture that bug. If your PR adds new behavior, make sure that behavior is fully tested. Every PR *must* include associated tests. (See [Unit tests](#unit-tests) for more.)
 - Submit your PR, making sure it references the issue you created.
 - If your PR fixes a bug, make sure the issue includes clear steps to reproduce the bug so we can test your fix.
 
 If you've completed all of these steps the core team will do its best to respond to the PR as soon as possible.
 
-### Contributing Code to Elements
+#### Contributing Code to Elements
 
 Though the aim of the Polymer library is to allow lots of flexibility and not get in your way, we work to standardize our elements to make them as toolable and easy to maintain as possible.
 
 All elements should follow the [Polymer element style guide](http://polymerelements.github.io/style-guide/), which defines how to specify properties, documentation, and more. It's a great guide to follow when building your own elements as well, for maximum standardization and toolability. For instance, structuring elements following the style guide will ensure that they work with the [`iron-component-page`](https://github.com/polymerelements/iron-component-page) element, an incredibly easy way to turn any raw element directly into a documentation page.
 
-### Contributing Code to the Polymer library
+#### Contributing Code to the Polymer library
 
 We follow the most common javascript and HTML style guidelines for how we structure our code - in general, look at the code and you'll know how to contribute! If you'd like a bit more structure, the [Google Javascript Styleguide](https://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml) is a good place to start.
 
 Polymer also participates in Google's [Patch Rewards Program](http://www.google.com/about/appsecurity/patch-rewards/), where you can earn cold, hard cash for qualifying security patches to the Polymer library. Visit the [patch rewards page](http://www.google.com/about/appsecurity/patch-rewards/) to find out more.
+
+## Unit tests
+
+All Polymer projects use [`web-component-tester`](https://github.com/Polymer/web-component-tester) for unit tests.
+The [`polyserve`](https://github.com/PolymerLabs/polyserve) utility is helpful for running tests in the browser.
+
+For maximum flexibility, install `web-component-tester` and `polyserve` locally:
+
+    npm install -g polyserve web-component-tester
+
+### Running the Polymer library unit tests
+
+To run the Polymer library unit tests:
+
+1.  Clone the [Polymer repo](https://github.com/polymer/polymer).
+
+2.  Install the dependencies:
+
+        npm install && bower install
+
+3.  Run the tests:
+
+        npm test
+
+    Or if you have `web-component-tester` installed locally:
+
+        wct
+
+You can also run individual test suites:
+
+<code>npm test <i>path/to/suite</i>
+
+Or:
+
+<code>wct <i>path/to/suite</i>
+
+For example:
+
+    npm test test/unit/template.html
+
+You can also run tests in the browser:
+
+        polyserve
+
+    Navigate to [`http://localhost:8080/components/polymer/test/runner.html`](http://localhost:8080/components/polymer/test/runner.html).
+
+### Running Polymer element unit tests
+
+To run the element unit tests, you need a global 
+install of `web-component-tester` or `polyserve` (or both). 
+
+1.  Clone the element repo.
+
+2.  Install the dependencies.
+
+        bower install
+
+3.  Run the tests:
+
+	    wct
+
+	Or run the tests in a browser:
+
+		polyserve
+
+	Navigate to:
+
+	<code>http://localhost:8080/components/<var>element-name</var>/test/runner.html</code>
+
+### Configuring `web-component-tester`
+
+By default, `web-component-tester` runs tests on all installed browsers. You can configure it
+to run tests on a subset of available browsers, or to run tests remotely using Sauce Labs.
+
+See the [`web-component-tester` README](https://github.com/Polymer/web-component-tester) for 
+information on configuring the tool. 
+
+