Carol/doc/doc 1221

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.

In the HTML page, this table requires me to scroll way to the right to read everything. I also feel like the numbering within the "Columns" row of the table doesn't quite fit the format of the table / deserves its own rendering. Maybe there's an alternate format that will make things a little more readable?