[1.6.x] Fixed #20794 -- Documented changes to validate_email

4e2e8f39d changed the way validate_email behaves for foo@localhost
email addresses, but wasn't listed in the release notes.
Backport of c928725b9 from master.

docs/releases/1.6.txt

@@ -797,6 +797,9 @@ Miscellaneous
   of the admin views. You should update your custom templates if they use the
   previous parameter name.
 
+* :meth:`~django.core.validators.validate_email` now accepts email addresses
+  with ``localhost`` as the domain.
+
 Features deprecated in 1.6
 ==========================