Adding context to MPPI readme

Signed-off-by: Steve Macenski <[email protected]>

nav2_mppi_controller/README.md

@@ -302,6 +302,8 @@ If you don't require path following behavior (e.g. just want to follow a goal po
 
 By default, the controller is tuned and has the capabilities established in the PathAlign/Obstacle critics to generally follow the path closely when no obstacles prevent it, but able to deviate from the path when blocked. See `PathAlignCritic::score()` for details, but it is disabled when the local path is blocked so the obstacle critic takes over in that state.
 
+If you want to slow further on approach to goal, consider increasing the ``threshold_to_consider`` parameters to give a hand off from the path tracking critics to the goal approach critics sooner - then tune those critics for your profile of interest.
+
 ### Prediction Horizon, Costmap Sizing, and Offsets
 
 As this is a predictive planner, there is some relationship between maximum speed, prediction times, and costmap size that users should keep in mind while tuning for their application. If a controller server costmap is set to 3.0m in size, that means that with the robot in the center, there is 1.5m of information on either side of the robot. When your prediction horizon (time_steps * model_dt) at maximum speed (vx_max) is larger than this, then your robot will be artificially limited in its maximum speeds and behavior by the costmap limitation. For example, if you predict forward 3 seconds (60 steps @ 0.05s per step) at 0.5m/s maximum speed, the **minimum** required costmap radius is 1.5m - or 3m total width.