Update 1.7 release notes re: User.get_absolute_url

`ABSOLUTE_URL_OVERRIDES` no longer works for the default auth.User model, as [explained here](http://stackoverflow.com/questions/25734384/accessing-user-get-absolute-url-in-django-1-7), and this should be reflected in the 1.7 release notes.

As a side note, a solution is proposed here: https://code.djangoproject.com/ticket/11775

I would think that this warrants a newly documented backwards incompatible change in 1.7, but figured would hold off on that to see what people have to say about this first.