Documentation missing endpoint explanation (#918)

* Documentation missing endpoint explanation

Add some information about GRAPHQL_URL. Otherwise people run into ERROR 400 problems, if they have a different endpoint.

* Update docs/testing.rst

Co-Authored-By: Jonathan Kim <jkimbo@gmail.com>

Co-authored-by: Jonathan Kim <jkimbo@gmail.com>

Author: fneitzel

docs/testing.rst

@@ -3,6 +3,8 @@ Testing API calls with django
 
 If you want to unittest your API calls derive your test case from the class `GraphQLTestCase`.
 
+Your endpoint is set through the `GRAPHQL_URL` attribute on `GraphQLTestCase`. The default endpoint is `GRAPHQL_URL = "/graphql/"`.
+
 Usage:
 
 .. code:: python