Add StackOverflow link to SUPPORT guidelines

SUPPORT.rst

@@ -5,13 +5,13 @@ Please refer to our documentation before asking questions:

http://django-wiki.readthedocs.io/

**DO NOT USE GITHUB FOR SUPPORT INQUIRIES! USE IRC OR MAILING LIST!**
* **DO NOT USE GITHUB FOR SUPPORT INQUIRIES!**
* **DO NOT EMAIL DEVELOPERS ASKING FOR SUPPORT!**

Django-wiki is community based, please try to be active. If you want help, plan to
give help, too. For instance, if you join IRC, then stay around and help others.
Django-wiki is community based. If you want support, then consider offering support to others. For instance, if you join IRC, then stay around and help others.

Please use our IRC or mailing list (google group) for getting in touch on development and support. Please do not email developers asking for personal support.

 * ``#django-wiki`` on irc.freenode.net
 * `django-wiki@googlegroups.com <https://groups.google.com/forum/#!forum/django-wiki>`__
Please use your preferred support channels for getting answers to your questions.

* `StackOverflow tag:django-wiki <https://stackoverflow.com/tags/django-wiki>`__
* ``#django-wiki`` on irc.freenode.net
* `django-wiki@googlegroups.com <https://groups.google.com/forum/#!forum/django-wiki>`__