Added some documentation explaining (1) that it's possible to add users via...

Added some documentation explaining (1) that it's possible to add users via the Django admin site, and (2) that in order to be able to add users via the admin site, you need to have both 'add user' and 'change user' permissions. Refs #9866

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9684 bcc190cf-cafb-0310-a4f2-bffc1f526a37