New chapter on discussion features for all users

@gwprice , @lou-wang , @marcotuts , @mhoeber , @srpearce , @WatsonEmily   : new chapter describing discussion forum features that can be used by both students and staff. Intention is to encourage exploration and identify features that may not be entirely evident in the UI (like the color coding of the dialog bubbles for posts) without over-documenting. 

An HTML version is available here: http://draft-discussions.readthedocs.org/en/latest/running_course/discussions_students.html
Hmm. I'm not sure where the line is between documenting and over-documenting (I know I tend to err on the side of the latter), but I think users will be confused if the text implies that all new posts have a dialog bubble and that's not the case in all situations. (I'm fairly computer literate, but I also get confused pretty easily, especially when I'm exploring a new UI.)