Fixed django#19628 - Noted that app for custom user model must be in INSTALLED_APPS

Thanks dpravdin and Jordan Messina.

Author: timgraham

docs/topics/auth/customizing.txt

@@ -404,8 +404,9 @@ the :setting:`AUTH_USER_MODEL` setting that references a custom model::
 
  AUTH_USER_MODEL = 'myapp.MyUser'
 
-This dotted pair describes the name of the Django app, and the name of the Django
-model that you wish to use as your User model.
+This dotted pair describes the name of the Django app (which must be in your
+:setting:`INSTALLED_APPS`), and the name of the Django model that you wish to
+use as your User model.
 
 .. admonition:: Warning