Add release notes for Review Board 2.5.1.

Review Request #7752 — Created Nov. 3, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x

Reviewers

This covers the bug fixes for Python 2.6 compatibility, webhooks, and
more, along with the new auto-population of the branch when posting
commits for review, and the changes to line numbers in the diff viewer.

Built the docs.