Fixed #12198 - CSRF changes not clearly noted in docs.

The docs no longer unhelpfully point to BackwardsIncompatibleChanges,
and instead a section has been added to help those upgrading and
those following trunk.  Tentative 1.2 release notes added.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11738 bcc190cf-cafb-0310-a4f2-bffc1f526a37