Fixed #15341 -- Clarified where translation problems should be reported in the...

Fixed #15341 -- Clarified where translation problems should be reported in the contributing docs. Thanks to bpeschier for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15590 bcc190cf-cafb-0310-a4f2-bffc1f526a37