AST node documentation

[nuno-silva]

I found it hard to find out what each Node's attribute mean. Some documentation would make it easier to use your project.

[viraptor]

Sorry, I inherited the project some weeks ago and wonder what the next steps will be.
To be honest, whatever time I'm going to put into it are unlikely to be documentation, but rather some polish and an official release.

I will happily merge any incoming documentation though.