Add release notes for Djblets 0.9.4.

Review Request #8452 — Created Oct. 10, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x
d55272e...

Reviewers

This covers the new DMARC support, extension unit testing additions,
and other improvements made since 0.9.3.

Built the docs and went through them, verifying that links worked
and the items matched what's in the commit history.