Add release notes for Review Board 2.5 RC 1.

Review Request #7593 — Created Aug. 16, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x

Reviewers

This covers all the changes we've made since beta 2. It also includes
all the fixes and improvements in 2.0.19 that haven't shipped yet.

Built the docs and read through them. All the links and references worked
(when built against /r/7592/).

Ran a spell checker and didn't see any typos.