Add to the documentation note about AUTH_USER_MODEL

[Dima Pravdin]

It's problem, that if I write the User model striсtly for documentation, I will get a validation error:

auth.user: Model has been swapped out for 'myapp.MyUser' which has not been installed or is abstract.

It's not enough just to set

AUTH_USER_MODEL = 'myapp.MyUser'

Beside that it's necessary add the 'myapp' in INSTALLED_APPS, but it's not specified in the documentation.

Sorry, if it's specified but I didn't find it :)

[jordanmessina]

One might argue that calling it a 'Django app' implies it's in the INSTALLED_APPS setting. However, I added a small note to the docs and created a pull request:
​https://github.com/django/django/pull/658

[Tim Graham <timograham@…>]

In 0375244eaeae1e2c09cc58c4c62e8f9e951217d0:

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

Thanks dpravdin and Jordan Messina.

[Tim Graham <timograham@…>]

In f6257a147df0be2f21c390ad10f6cd724b99cd93:

[1.5.x] Fixed #19628 - Noted that app for custom user model must be in INSTALLED_APPS

Thanks dpravdin and Jordan Messina.

Backport of 0375244eae from master

[Tim Graham <timograham@…>]

In f6257a147df0be2f21c390ad10f6cd724b99cd93:

[1.5.x] Fixed #19628 - Noted that app for custom user model must be in INSTALLED_APPS

Thanks dpravdin and Jordan Messina.

Backport of 0375244eae from master