Document how to set up nbgrader for multiple graders when running without JupyterHub

[jhamrick]

There is already documentation on how to use nbgrader with multiple graders with JupyterHub, but not when not using JupyterHub.

Briefly, the answer is that you still need access to a shared server, which would host the course directory. You would then run a password-protected version of the notebook on a public port on that server, and give the link to your graders so they can access the formgrader.

[perllaghu]

The route we use is to use an external database some external FileStore space - unique for each course.

Each Instructor then uses the appropriate db_url for the course database, and mount the external FileStore space into their directory somewhere.... Then set CourseDirectory.root to point to that FileStore directory