Add database session to the example

README.md

@@ -63,6 +63,21 @@ class Query(graphene.ObjectType):
 schema = graphene.Schema(query=Query)
 ```
 
+We need a database session first:
+
+```python
+from sqlalchemy import (create_engine)
+from sqlalchemy.orm import (scoped_session, sessionmaker)
+
+engine = create_engine('sqlite:///database.sqlite3', convert_unicode=True)
+db_session = scoped_session(sessionmaker(autocommit=False,
+                                         autoflush=False,
+                                         bind=engine))
+# We will need this for querying, Graphene extracts the session from the base.
+# Alternatively it can be provided in the GraphQLResolveInfo.context dictionary under context["session"]
+Base.query = db_session.query_property()
+```
+
 Then you can simply query the schema:
 
 ```python