MPC README documents the previous controller #175

The README of easynav_mpc_controller was not updated with the LTV-MPC rewrite (#169). It lists parameters that no longer have any effect (safety_radius, use_collision_checker, verbose), the old default horizon (5 steps, now 20) and the old topics. It does not mention motion_model or most of the current parameters (weights, time_constant, obstacles, goal tolerances, OSQP settings).