Fixed #9067: Added a note that you can register a model with the admin without...

Fixed #9067: Added a note that you can register a model with the admin without providing a ModelAdmin object. Thanks to Rob Hudson for the suggestion.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9024 bcc190cf-cafb-0310-a4f2-bffc1f526a37