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.
Use comments to skip tests instead of a tag
Merge pull request #1005 from edx/jtauber/link-to-how-to-contribute

Include / Link to How To Contribute wiki page
Merge pull request #1143 from edx/will/skip-tests-with-comments

Use comments to skip tests instead of a tag
Merge remote-tracking branch 'upstream/master' into cheng-shiwen/master
fix the video caption lag issue