Added a warning about nonexistent FK constraints when unmigrated apps depend on migrated ones.

Be aware, however, that unmigrated apps cannot depend on migrated apps, by the

very nature of not having migrations. This means that it is not generally

possible to have an unmigrated app have a ForeignKey or ManyToManyField to

a migrated app; some cases may work, but it will eventually fail.

possible to have an unmigrated app have a ``ForeignKey`` or ``ManyToManyField``

to a migrated app; some cases may work, but it will eventually fail.

.. warning::

    Even if things appear to work with unmigrated apps depending on migrated

    apps, Django may not generate all the necessary foreign key constraints!

This is particularly apparent if you use swappable models (e.g.

``AUTH_USER_MODEL``), as every app that uses swappable models will need