Release notes for 7-23-2014

@mhoeber @lamagnifica Please review. I only found three items to include in the notes this week.

Note that I have an internal comment on line 26.

Draft at http://draft-release-notes-7-23.readthedocs.org/en/latest/07-23-2014.html
Added the Participating in Course Discussions chapter.  wasn't in last one