This patch standardizes the CONTRIBUTING.rst file and adds the
required doc/source/contributor/contributing.rst
Change-Id: I3f7ee29094085f1abefacd75f44a16fb7e679a82
Story: #2007236
Task: #38523
Moves the Release Notes section to its own page to give it a higher
level heading to make it easier to find. Also updates info about
the Glance team's policy about the "prelude" section.
Change-Id: I31351068124e0981519ccc2e33a7bdc16f8197fd