Fixed #8326: added signals documentation.

Thanks to MercuryTide's signal whitepaper from 2006 (http://www.mercurytide.co.uk/whitepapers/django-signals/) for inspiration and ideas for organization, and to the folks who got the Signals wiki page together for some of the content.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8590 bcc190cf-cafb-0310-a4f2-bffc1f526a37