[1.1.X] Document the fact that we now put dead branches in the attic. Backport...

[1.1.X] Document the fact that we now put dead branches in the attic. Backport of r11523 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@11524 bcc190cf-cafb-0310-a4f2-bffc1f526a37