Fixed #6740 -- Documented in docs/flatpage.txt that the flatpages app requires...

Fixed #6740 -- Documented in docs/flatpage.txt that the flatpages app requires the sites framework. Thanks, mail@dirkmeister.de

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7208 bcc190cf-cafb-0310-a4f2-bffc1f526a37