Use rst instead of md for a rst file

CONTRIBUTING.rst

@@ -4,11 +4,24 @@
 Contributing
 ============
 
+If you're not sure this is the correct place to file an issue
+then please file an issue on the `mozilla/addons`_ project instead.
+
 Before contributing code, please note:
 
-* You agree to license your contributions under the [license](https://github.com/mozilla/addons-server/blob/master/LICENSE).
-* Ask on the [dev-addons mailing list](https://mail.mozilla.org/listinfo/dev-addons) or in GitHub before making changes.
-* Follow [PEP8](http://www.python.org/dev/peps/pep-0008/), [jshint](http://www.jshint.com/) and our other [style guide conventions](http://mozweb.readthedocs.org/en/latest/index.html).
-* Please write tests and read the docs on [addons-server](http://addons-server.readthedocs.org/en/latest/).
+-  You agree to license your contributions under the `license`_.
+-  Please ask on the `dev-addons mailing list`_ before submitting
+   pull-requests for new features or large changes that are not related to
+   existing issues.
+-  Follow `PEP8`_, `jshint`_ and our other `style guide conventions`_.
+-  Please write tests and read the docs on `addons-server`_.
 
-And thank you for contributing code.
+Thank you for contributing!
+
+.. _license: https://github.com/mozilla/addons-server/blob/master/LICENSE
+.. _dev-addons mailing list: https://mail.mozilla.org/listinfo/dev-addons
+.. _PEP8: https://www.python.org/dev/peps/pep-0008/
+.. _jshint: http://www.jshint.com/
+.. _style guide conventions: https://mozweb.readthedocs.org/en/latest/
+.. _addons-server: https://addons-server.readthedocs.org/en/latest/
+.. _mozilla/addons: https://github.com/mozilla/addons/issues/new