add GSSAPI: prefix to warning message

src/django_gssapi/backends.py

@@ -33,7 +33,7 @@ logger = logging.getLogger('django_gssapi')
 
 class GSSAPIBackend(object):
     def authenticate(self, request, gssapi_name):
-        warnings.warn('example backend do not use in production!')
+        warnings.warn('GSSAPI: example backend do not use in production!')
         # pylint: disable=invalid-name
         User = get_user_model()
         try: