Issue #712. Add docs around fixtures to CONTRIBUTING.md

CONTRIBUTING.md

@@ -24,6 +24,7 @@ You are welcome to contribute to this project. Here are the guidelines we try to
 * [Coding](#coding)
 * [Running Tests](#running-tests)
   * [Functional Tests](#functional-tests)
+  * [Functional Tests using Fixture Data](#functional-tests-using-fixture-data)
 * [Writing Tests](#writing-tests)
   * [Python Unit Tests](#python-unit-tests)
   * [JS Functional Tests](#js-functional-tests)
@@ -391,6 +392,33 @@ To run a single test suite, where foo.js is the file found in the `tests/functio
 node_modules/.bin/intern-runner config=tests/intern functionalSuites=tests/functional/foo.js user=testusername pw=testpassword
 ```
 
+## Functional Tests using Fixture Data
+
+It's possible to mock the communications with GitHub API servers using local fixture data. To run tests using these mocked repsonses, run the server in "test mode":
+
+```bash
+python run.py -t
+```
+
+You can then run intern tests or do local development and the files in the `/tests/fixtures/` directory will be served as responses.
+
+## Adding Fixtures
+
+To indicate that the app should send fixture data, use the `@mockable_response` decorator for an API endpoint.
+
+If the endpoint you are trying to mock has `GET` parameters, you will need to create a file that has the `GET` parameters encoded in the filename. The source of `@mockable_repsonse` explains how this is done:
+
+```python
+if get_args:
+    # if there are GET args, encode them as a hash so we can
+    # have different fixture files for different response states
+    checksum = hashlib.md5(json.dumps(get_args)).hexdigest()
+    file_path = FIXTURES_PATH + request.path + "." + checksum
+    print('Expected fixture file: ' + file_path + '.json')
+```
+
+You can look at the server console's `Expected fixture file:` message to know what file it is expecting.
+
 ## Writing Tests
 
 Contributions that add or modify major functionality to the project should typically come with tests to ensure we're not breaking things (or won't in the future!). There's always room for more testing, so general contributions in this form are always welcome.