Document 4-wheel steering controller

[matthew-reynolds]

If possible, it would be great to have some documentation on 4-wheel steering controller.
https://github.com/ros-controls/ros_controllers/blob/melodic-devel/four_wheel_steering_controller/src/four_wheel_steering_controller.cpp
Specifically, derivation or description of the equations for calculating wheel velocities/angles, and what the difference is between twist command block (line 375) vs 4ws command block (line 418).

Originally posted by @ros-gy in #467 (comment)

[matthew-reynolds]

See #55 and #280 for the original discussion and PR adding this package.

@vincentrou As the original author of the four_wheel_steering_controller, would you be able to help out with the documentation or point us to some resources?