Document pure Apollo implementation

[StefanSmith]

This is possibly a strange issue to raise since it emphasises not using this library but since the README contains instructions for Using Authorization and Subscription links with Apollo Client (No offline support), I thought it might be helpful to also document how to interact with AppSync using only Apollo libraries. Since the AppSync websocket subscriptions workflow was documented recently, this is now possible.

I have raised an issue on the Apollo project to document an example implementation: apollographql/apollo-feature-requests#224