Add help docs listing the expected csv column names. Closes #808.
Add help docs listing the expected csv column names. Closes #808.

w/ @jhamrick

jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader