Document how to use jOOQ-checker with Java 8 - 11

[lukaseder]

With #5899, we've introduced support for the ErrorProne framework, in addition to the checker framework. Unfortunately, both frameworks are picky with what JDK versions they work with. The checker framework is based on JDK 8 (and regrettably doesn't support newer JDKs). The ErrorProne framework is based on JDK 9, can be made to work with JDK 8, but doesn't seem to support newer JDKs. We should definitely also have a separate section about how to use the ErrorProne framework.

The burden of figuring out how to configure these lies with the user. In addition to that, the jOOQ Open Source Edition 3.12 is based on JDK 11, whereas the jOOQ Open Source Edition 3.11 is based on JDK 8. (Commercial distributions exist for JDK 6 and 8)

This definitely needs documentation.