Update README to include reference of a GraphQLController

[Michannne]

User has been confused about app not working
No documentation showing in GraphiQL and Intellisense not working in GraphiQL

Problem was that user did not have a GraphQLController to handle requests

Expected Outcome
User should know to add GraphQLController to add references
Readme should reference the need to add a GraphQLController to their project
(Maybe this can be simplified?)

[Michannne]

@Epsilekt Yes it should be possible

[Michannne]

@Tim-Maes I'm not able to get the project details from the user.