Added release notes for Djblets 0.8.26.

Review Request #8122 — Created April 13, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.8.x
70af3b1...

Reviewers

This covers the improvements made to the extensions framework for
includes and default bundles, as well as the fixes for metadata
processing, and the payload output issues for config forms.

Built the docs, read through them, and checked links.