Update README to include reference of a GraphQLController

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?)

Start by reviewing the README and the existing GraphiQL-related guidance. Add a clear reference that projects need a GraphQLController to handle requests and support GraphiQL documentation and IntelliSense, then confirm the README explains the expected setup without requiring unstated project changes.

Written by the indexing model from the issue text.