Adds sample student communications (email msgs)

@mmacfarlane , @WatsonEmily , please review and provide many opinions on what to add/delete/change. An html version is available here: http://draft-student-communications.readthedocs.org/en/latest/releasing_course/course_launching.html

@mhoeber , @srpearce please copy edit and proofread.
leaving this alone for now as making this single change would make the appearance of the value inconsistent with the rest of the description. (this shows up as a change in the file because of line wrapping only)