Prepare to release Djblets 0.8.20.

Review Request #7430 — Created June 15, 2015 and submitted — Latest diff uploaded

Information

Djblets
release-0.8.x
1e9f70f...

Reviewers

This adds release notes for the new version of Djblets, which fixes a
problem in modifying data returned from WebAPIResource.serialize_object.

This also bumps the version and release flag. With this change, we're
good to release.

Built the docs and viewed them.