add help msg for plugin class
add help msg for plugin class

ellisonbg/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader