First of all, thank you for taking the time to contribute!!
We put our best effort into providing high quality software that is readable (class, field, and method names are written in English as much as possible), well documented, and well tested. We would greatly appreciate if your contribution goes the same way.
We use a custom Java formatting that can be found here. It can be easily integrated to different IDEs such as Eclipse and IntelliJ. Any contribution we'll be formatted before being accepted.
You can report to us any misleading, incomplete documentation or any typos you find. This is the easiest way to contribute, but it is still an important contribution that we would greatly appreciate.
If you find a bug, it will greatly help us if you could provide a failing JUnit test with the bug report or at least a concrete example so it is easy for us to reproduce the bug on our side.