Fixed #13184 -- Document the requirement that custom model fields using...

Fixed #13184 -- Document the requirement that custom model fields using SubfieldBase should probably implement formfield. Thanks to Mark L. for the report, and to Joseph and Honza for the guidance on the solution.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12979 bcc190cf-cafb-0310-a4f2-bffc1f526a37