Create integration guide

[dk1844]

Once the gateway is usable (not necessarily all possible options available, but at least simple in-memory auth with fixed user set), INTEGRATION.md or equivalent page should be created and linked from the README.md.

Here, we should advice potential users of this project how to (deploy and) integrate with existing tools that do not yet have any form of authentication.

A code snipet with a concrete example might be good.
Optionally, Spring-security integration example may be specifically mentioned, because this is our prevaling framework in AbsaOSS.

[TheLydonKing]

Can we consider this issue closed since the library exists and has it's own readme on how to use it?