Add in the docs that the plugin is Guice dependent

[UnconventionalMindset]

Came out after this: https://stackoverflow.com/a/51459491/4715008 If people are not using Guice they should know this library is not going to work or what is the workaround to it.

[gmethvin]

We use this with compile-time DI and it works fine. The main issue is that the controller relies on global state, so you need to create an instance of the SwaggerPluginImpl on startup to initialize that global state for Swagger.

In https://github.com/Iterable/swagger-play/ we fixed some of the global state issues.