Add release notes for Review Board 3.0.8.

Review Request #10055 — Created July 2, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
1982b62...

Reviewers

This covers the new, full support for RB Gateway (as of the 1.0 release
coming out), as well as bug fixes for search filtering, Subvertpy, and
draft review deletion.

Built the docs and checked for build and spelling errors.