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.

Maybe you could say something along the lines of "... at least one column to identify the user (either "email" or "username") and a "cohort" column to identify the cohort to which they should be added".