This Wiki provides some of the baisc concepts of the engine and some Examples too of course. The content discribes some core aspects of the engine. You will find a lot of useful classes next to the one discribed here. Check out each package to find a README.md file which discribes the package.
Not much to tell you here. Every class has a short legal text. It's a BSD license so go and use or modify the engine. I even want you to contribute :)