First draft
Edits per review comments
Fix formatting error after rebase
Updates per reviews, further conversations

Also removed redundant note about username and email checking
Fix wording re comma requirement in csv
Changes to column requirements description
Further edits from review comments
Further edits from review comments
Add note: row by row processing

Affects count of Students Added
Minor fix
Fix extra spaces
Fix description of Students Not Found in results
Merge pull request #45 from edx/carol/doc/DOC-1221

Assign students to cohorts by uploading a csv file