Update the release notes for Djblets 0.9 beta 2.

Review Request #7533 — Created July 20, 2015 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x

Reviewers

This adds the new changes made since the release notes were written,
fleshes out a few bits, and adds cross-referencing to the Djblets
codebase documentation.

Built the docs and inspected the output. The display of the content
looked fine, and all cross-references (except for those not yet deployed
on the site or some other special ones that Sphinx isn't indexing properly)
were working.