Documenting the difference in quaternion representation between MuJoCo and Drake

[akhilsathuluri]

API differences need to be noted when converting rotations between the simulator and the controller. For example,

• MuJoCo API uses an [x, y, z, w] quaternion representation as shown here
• Whereas, Drake uses the [w, x, y, z] ordering as described here

[traversaro]

See also https://github.com/clemense/quaternion-conventions for a general reference on quaternion conventions.