Add release notes for Review Board 2.5.7.

Review Request #8362 — Created Aug. 29, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
67de132...

Reviewers

This covers all the improvements made since 2.5.6.1. Some of the
highlights include:

* New user selector
* E-mail improvements
* Settings access controls for staff members
* Fixes for custom fields and rich text
* Fixes for WebHooks
* Fixes for Local Sites
* Fixes CVS, Bitbucket, and GitLab

Built the docs. Sphinx didn't complain about any references, syntax,
or links.

Manually checked the links and references.

Ran spell check.