Add manual for setting up target Java edition

Actually I'm looking at the documentation and cannot find any method of setting up the Java version; e.g., I want to parse (maybe) Java 7, then... how to?. I'm beginner, so of course I'm sorry for this annoying issue.

Could you guys put something about that inside the manual?

[matozoid]

Here's the missing manual: https://matozoid.github.io/2017/04/11/enable-java-9-support.html https://javaparser.org/setting-java-8-9-10-etc/

[matozoid]

We could put that in the book though.

[matozoid]

@SmiddyPence - did you put it in the book?

@matozoid So I guess validators aren't priorized for now in the book, right?

[matozoid]

@hydroper sadly the book is the part of this project that gets the least attention :-/