Add a new README for the Review Board codebase.

Review Request #7382 — Created June 5, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
4be710f...

Reviewers

Our old README for Review Board was pretty small and useless. Not a
great landing experience for users going to the project on GitHub. This
new Markdown-formatted README covers the history and purpose of
Review Board, points to the installation pages, documentation, Power
Pack, RBTools, support information, Happy Users page, and basic
information on how to contribute to the project.

Viewed this in Marked (a Markdown renderer that can render as GitHub would).
Things looked fine.