Add release notes for Review Board 2.0.12.

Review Request #6676 — Created Dec. 5, 2014 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
302a268...

Reviewers

2.0.12 was a large release, with all the new Markdown improvements, API
improvements, and a large number of bug fixes. This covers all the
changes in detail.

Built the docs. Didn't see any errors, other than the hosting-service-hook
reference missing (due to the referenced docs not yet being up on the site).

Viewed the docs. Everything looked fine.