Link to How To Contribute wiki page

Created a CONTRIBUTING.rst which GitHub will pick up and included a
link to the How To Contribute wiki page there. Also added the same link
to the README in the "How To Contribute" section.

CONTRIBUTING.rst

+How To Contribute
+=================
+
+Contributions are very welcome.
+
+Please read `How To Contribute <https://github.com/edx/edx-platform/wiki/How-To-Contribute>`_ for details.

README.md

@@ -345,9 +345,9 @@ with `overview.md` to get an introduction to the architecture of the system.
 How to Contribute
 -----------------
 
-Contributions are very welcome. The easiest way is to fork this repo, and then
-make a pull request from your fork. The first time you make a pull request, you
-may be asked to sign a Contributor Agreement.
+Contributions are very welcome.
+
+Please read [How To Contribute](https://github.com/edx/edx-platform/wiki/How-To-Contribute) for details.
 
 Reporting Security Issues
 -------------------------