Document ragg2 and `g` commands

[XVilka]

Basics on how to use it, simple and C syntax, couple examples

[kuqadk3]

Sorry, but I couldn't find any 'g' command in ragg2.c source code

https://github.com/radareorg/radare2/blob/master/libr/main/ragg2.c

[XVilka]

@kuqadk3 what I meant is the g command in the radare2 itself, it's the command that exposes ragg2 features but inside the radare2 session.