update db.rst

expand on Django ORM basics

docs/scenarios/db.rst

@@ -33,3 +33,11 @@ to provide database access.
 It's based on the idea of `models <https://docs.djangoproject.com/en/1.3/#the-model-layer>`_, an abstraction that makes it easier to
 manipulate data in Python.
 
+The basics:
+
+- Each model is a Python class that subclasses django.db.models.Model.
+- Each attribute of the model represents a database field.
+- Django gives you an automatically-generated database-access API; see `Making queries <https://docs.djangoproject.com/en/dev/topics/db/queries/>`__.
+to provide database access.
+
+