added documentation
added documentation

wellcaffeinated/PhysicsJS,jpellerin/PhysicsJS,VANITAX/PhysicsJS,Nayar/PhysicsJS,Nayar/PhysicsJS,Rawneed/PhysicsJS,Rawneed/PhysicsJS,wellcaffeinated/PhysicsJS,eric-seekas/PhysicsJS,Luwdo/PhysicsJS,Luwdo/PhysicsJS,haithemaraissia/PhysicsJS,131/PhysicsJS,aldeed/PhysicsJS,griever989/PhysicsJS,rohit765/PhysicsJS,jpellerin/PhysicsJS,pietro909/PhysicsJS,eric-seekas/PhysicsJS,rohit765/PhysicsJS,VANITAX/PhysicsJS,mcanthony/PhysicsJS,pietro909/PhysicsJS,mcanthony/PhysicsJS,haithemaraissia/PhysicsJS,griever989/PhysicsJS,aldeed/PhysicsJS