Added release note for PBKDF2 iteration count increase.

* :class:`~django.contrib.auth.forms.PasswordResetForm` now

  has a method :meth:`~django.contrib.auth.forms.PasswordResetForm.send_email`

  that can be overridden to customize the mail to be sent.

* The ``max_length`` of :attr:`Permission.name

  <django.contrib.auth.models.Permission.name>` has been increased from 50 to

  255 characters. Please run the database migration.

* :attr:`~django.contrib.auth.models.CustomUser.USERNAME_FIELD` and

  :attr:`~django.contrib.auth.models.CustomUser.REQUIRED_FIELDS` now supports

  :class:`~django.db.models.ForeignKey`\s.

* The default iteration count for the PBKDF2 password hasher has been

  increased by 33%. This backwards compatible change will not affect users who

  have subclassed ``django.contrib.auth.hashers.PBKDF2PasswordHasher`` to

  change the default value.

:mod:`django.contrib.gis`

^^^^^^^^^^^^^^^^^^^^^^^^^^