[1.7.x] Added admonition about reusable apps and AUTH_USER_PROFILE.

   have a ForeignKey to each other and seeing how ``makemigrations`` resolves that

   circular dependency if you want to see how it's usually done)

.. admonition:: Reusable apps and ``AUTH_USER_MODEL``

   Reusable apps shouldn't implement a custom user model. A project may use

   many apps, and two reusable apps that implemented a custom user model

   couldn't be used together. If you need to store per user information in your

   app, use a :class:`~django.db.models.ForeignKey` or

   :class:`~django.db.models.OneToOneField` to ``settings.AUTH_USER_MODEL``

   as described below.

Referencing the User model

--------------------------