Recommendation: add SQLAlchemy session object context_value in README

[Diggitysc]

Thank you for creating such a great library. This is a huge benefit to my project.

That being said, I had to jump through a few hoops to figure out how to get querying up and running for the graphiql interface (with SQLalchemy running the backend models and linked into the database session/transaction manager)

I was finally able to get everything wired up by setting the following:
return serve_graphql_request(request, schema, context_value={'session': request.dbsession}, graphiql_enabled=True)

It would be helpful to list for other SQLAlchemy users who want a quick graphql interface via webob instead of flask.

Thank you guys for all your hard work

[sydoluciani]

If cookie cutter has been used to create pyramid project,
and result of session factory(new session) is added to request method as dbsession, then we should replace session with dbsession:
serve_graphql_request(request, schema, context_value={'session': request.dbsession}, graphiql_enabled=True)

https://github.com/Pylons/pyramid-cookiecutter-starter/blob/latest/%7B%7Bcookiecutter.repo_name%7D%7D/%7B%7Bcookiecutter.repo_name%7D%7D/sqlalchemy_models/__init__.py#L75

Otherwise, session is available in request method with the same name and current example works fine:
https://github.com/graphql-python/webob-graphql#pyramid