Added release notes for today's app-loading improvements.

Improvements thus far include:

* Applications can run code at startup, before Django does anything else, with

  the :meth:`~django.apps.AppConfig.setup` method of their configuration.

* It is possible to omit ``models.py`` entirely if an application doesn't

  have any models.

* The name of applications can be customized in the admin with the

  :attr:`~django.apps.AppConfig.verbose_name` of application configurations.

* Django imports all application configurations and models as soon as it

  starts, through a deterministic and straightforward process. This should

  make it easier to diagnose import issues such as import loops.

New method on Field subclasses

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