Add CONTRIBUTING file

CONTRIBUTING.rst

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+Contributing
+============
+
+Below is a list of tips for submitting issues and pull requests.  These are
+suggestions and not requirements.
+
+Submitting Issues
+-----------------
+
+Issues are often easier to reproduce/resolve when they have:
+
+- A pull request with a failing test demonstrating the issue
+- A code example that produces the issue consistently
+- A traceback (when applicable)
+
+Pull Requests
+-------------
+
+When creating a pull request, try to:
+
+- Write tests if applicable
+- Update the `README`_ file if needed
+- Update the documentation if needed
+
+.. _README: README.rst
+
+Testing
+-------
+
+Please add tests for your code and ensure existing tests don't break.  To run
+the tests against your code::
+
+    python setup.py test
+
+Please use tox to test the code against supported Python and Django versions.
+First install tox::
+
+    pip install tox
+
+To run tox and generate a coverage report (in ``htmlcov`` directory)::
+
+    make test
+
+Generating documentation
+------------------------
+
+To regenerate the documentation use::
+
+    make docs
+
+The generated documentation HTML files can be found in ``docs/_build/html``.