Add database session to the example

Coming from https://docs.graphene-python.org/projects/sqlalchemy/en/latest/tutorial/ as a python noob I failed to run their example but could fix this example by adding the database session.

Author: clemens-tolboom

README.md

@@ -58,6 +58,18 @@ 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))
+```
+
 Then you can simply query the schema:
 
 ```python