Add examples for validation rules

Author: kiendang

Diff for: docs/index.rst

@@ -33,5 +33,6 @@ For more advanced use, check out the Relay tutorial.
    authorization
    debug
    introspection
+   validation
    testing
    settings

Diff for: docs/validation.rst

@@ -0,0 +1,29 @@
+Query Validation
+================
+
+Graphene-Django supports query validation by allowing passing a list of validation rules (subclasses of `ValidationRule <https://github.com/graphql-python/graphql-core/blob/v3.2.3/src/graphql/validation/rules/__init__.py>`_ from graphql-core) to the ``validation_rules`` option in ``GraphQLView``.
+
+.. code:: python
+
+    from django.urls import path
+    from graphene.validation import DisableIntrospection
+    from graphene_django.views import GraphQLView
+
+    urlpatterns = [
+        path("graphql", GraphQLView.as_view(validation_rules=(DisableIntrospection,))),
+    ]
+
+or
+
+.. code:: python
+
+    from django.urls import path
+    from graphene.validation import DisableIntrospection
+    from graphene_django.views import GraphQLView
+
+    class View(GraphQLView):
+        validation_rules = (DisableIntrospection,)
+
+    urlpatterns = [
+        path("graphql", View.as_view()),
+    ]