Fixed #15055 -- added information about (and an example of) the csrf_token...

Fixed #15055 -- added information about (and an example of) the csrf_token template tag to the forms documentation. Thanks to sneakyness for the report and bpeschier for the draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15445 bcc190cf-cafb-0310-a4f2-bffc1f526a37