Clean up CONTRIBUTING.md

Summary: CONTRIBUTING.md was mostly a duplicate of a section in README.md. I
deduped the content, cutting the README.md section down to a link to
CONTRIBUTING.md. I also reordered and cleaned up some of the text in
CONTRIBUTING.md.

Closes #3205
Created from: https://github.com/facebook/hhvm/pull/3205

Reviewed By: @ptarjan

Differential Revision: D1439724