Django admin documentation

[justinlittman]

As suggested by @chrisprom, provide better documentation on Django admin interface, especially user and group administration.

Note that there is some documentation in http://sfm.readthedocs.io/en/latest/authentication.html. However, it is neither complete or easy to find.

[justinlittman]

Also, document that users should be added via app, not admin.

[lwrubel]

This was also suggested by Alexandra on the DocNow project.