docs: Add database session to the example (#249)

* 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.

* Update README.md

---------

Co-authored-by: Erik Wrede <erikwrede@users.noreply.github.com>

Author: clemens-tolboom

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