Add a basic DEVELOPMENT file to explain common scenarios (#1871)

DEVELOPMENT.md

@@ -0,0 +1,43 @@
+## Environment
+
+Requires:
+
+- [Yarn](https://yarnpkg.com/)
+- Node 10.x
+
+## Building
+
+The SDK uses [Webpack](https://webpack.js.org/) to compile all JavaScript assets into a set of output modules to be consumed by other module builders such as Webpack and [Rollup](https://rollupjs.org/guide/en/), or directly into and HTML file via the CDN.
+
+To perform a build, use the `build` script:
+
+```
+yarn build
+```
+
+## Running Tests
+
+Unit tests can be executed using [Jest](https://jestjs.io/) by issuing the following command:
+
+```
+yarn test:jest
+```
+
+To interactively perform tests using Jest's `watch` mode, use:
+
+```
+yarn test:jest:watch
+```
+```
+
+## The SDK Playground
+
+To test the SDK manually and play around with the various options and features, you can invoke the Playground by using:
+
+```
+yarn start
+```
+
+Next, open `https://localhost:3000/support`, which will display a simple web app that allows you to interact with Auth0 to test functionality. The HTML template in `support/index.html` can be modified to test various different pieces of functionality.
+
+This is preconfigured with an Auth0 tenant and client ID but you may change this to your own for testing.