Fixed #14455 -- Documented the backwards compatibility policy for local flavors. Implemented the policy for the changes in the Indonesian local flavor (from r14195) that stimulated the development of this policy. Thanks to Karen, Alex, Ramiro and Chris for their help developing the policy.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14411 bcc190cf-cafb-0310-a4f2-bffc1f526a37