This patch standardizes the CONTRIBUTING.rst file and adds the
required doc/source/contributor/contributing.rst
Swift already had a detailed CONTIRBUTING.rst and an informative
REVIEW_GUIDELINES.rst in the root of the repo. So we are also pulling
them into the contributor documentation so they can not only be easily
found in the checked repo but in the online documentation.
Change-Id: I4c84efbe50eb25ab922c9d6b69198dae341af48b
