Fixed #7996: Added a note that you need to register your site with Google in...

Fixed #7996: Added a note that you need to register your site with Google in order for the ping_google command to work. Thanks to Jason Broyles (issya) for the suggestion.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8519 bcc190cf-cafb-0310-a4f2-bffc1f526a37