Add release notes for Review Board 3.0 beta 2.

Review Request #9243 — Created Oct. 3, 2017 and submitted — Latest diff uploaded

Review Board

3.0 Beta 2 is a large release bringing new update/discussion
highlighting for review requests and reviews, the ability to revoke
Ship-Its, issue verification, Emoji, OAuth2 provider support, on-the-fly
search indexing, new icons, rate limiting, new extension and API
abilities, performance improvements, and a lot of bug fixes.

This change adds release notes for this release, complete with
screenshots for some of the key features.

Checked for spelling errors.

Built the docs and verified there were no build errors and that all links
and references worked.