Add release notes for RBTools 0.7.4.

Review Request #7386 — Created June 5, 2015 and submitted — Latest diff uploaded

Information

RBTools
release-0.7.x
9798b89...

Reviewers

There have been some fixes for Python compatibility issues, TFS,
Mercurial, and rbt post. These release notes cover all these changes in
preparation for a 0.7.4 release.

Built the docs and read through them.