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.
Moved to a more comprehensive contributing doc.

Based on the How To Contribute wiki page (and switched to markdown so
it's a simple copy-paste).
Link to wiki with absolute URLs
Added linebreaks.
Merge pull request #1005 from edx/jtauber/link-to-how-to-contribute

Include / Link to How To Contribute wiki page