Add release notes for Review Board 2.0.20.

Review Request #7624 — Created Sept. 13, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x

Reviewers

These cover all the e-mail, diff viewer, file attachment, etc. fixes
that have gone into this release.

It also contains a variation on the prior security announcement,
instructing people on how to install Django 1.6.11.1.

Built the docs and read through them.