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.
I would also add something about students who have never taken a MOOC reviewing edx101 and providing the link, so they can become acquainted with our platform and its features.