Improve docs.

docs/sections/contribute.rst

@@ -21,6 +21,9 @@ Use `tox <https://pypi.python.org/pypi/tox>`_ for running tests in each of the e
     # Run with Python 2.7 and Django 1.9.
     $ tox -e py27-django19
 
+    # Run single test file.
+    $ python runtests.py oidc_provider.tests.test_authorize_endpoint
+
 If you have a Django project properly configured with the package. Then just run tests as normal::
 
     $ python manage.py test --settings oidc_provider.tests.app.settings oidc_provider