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.

Ditto. Perhaps describe the two options individually? A two column csv with the first column for email address or username and the second for... A three column csv with a column for email, a column for username, and a column for....