Assign students to cohorts by uploading a csv file

First draft of doc: course authors can use a csv upload to assign students to cohorts.
Will continue to update if additional info surfaces about UTF-8 and other requirements/limitations.

When I first read this, I read it as, you can have one column, with either usernames or emails in it, which isn't right. You may want to make it clear that, you must have one or both of the "email" and "username" columns, and the cohort column.