Document setting up library with Spring, without using spring boot

[kjc-heliosx]

It's unclear whether this project supports configuration manually, without running in a Spring Boot environment. I'll be able to work it out by digging out what classes I need to add to my component scanning, though this ought to be documented.

[blevine]

I'd be very interested in your findings on this as I am looking to use spring-graphql to implement a GraphQL API in an existing non-Spring server. I like many of the features of spring-graphql, but I need to access them without using Spring Boot

[bclozel]

I've added a new section in the reference documentation for standalone setups.