Lower threshold for java newbies

[tillwenke]

without a bullet-proof java/mvn setup on ubuntu it was tough for me to get started.

a way how to get it running without a hassle should be stated in the readme.

eventually got it working with:

https://docs.aws.amazon.com/corretto/latest/corretto-11-ug/generic-linux-install.html (v17) JAVA_HOME="/usr/lib/jvm/java-17-amazon-corretto" bash --posix mvnw clean compile package (posix!)