add documentation for terminal lane change path

Signed-off-by: mohammad alqudah <[email protected]>

planning/behavior_path_planner/autoware_behavior_path_lane_change_module/README.md

@@ -874,6 +874,16 @@ endif
 @enduml
 ```
 
+## Terminal Lane Change Path
+
+Depending on the space configration around the Ego vehicle, it is possible that a valid LC path cannot be generated. If that happens, then Ego will get stuck at `terminal_start` and will be not be able to proceed. Therefore we introduced the terminal LC path feature; when ego gets near to the terminal point (dist to `terminal_start` is less than the maximum lane change length) a terminal lane changing path will be computed starting from the terminal start point on the current lane and connects to the target lane. The terminal path only needs to be computed once in each execution of LC module. If no valid candidate paths are found in the path generation process, then the terminal path will be used as a fallback candidate path, the safety of the terminal path is not ensured and therefore it can only be force approved. The following images illustrate the expected behavior without and with the terminal path feature respectively:
+
+![no terminal path](./images/lane_change-no_terminal_path.png)
+
+![terminal path](./images/lane_change-terminal_path.png)
+
+Additionally if terminal path feature is enabled and path is computed, stop point placement can be configured to be at the edge of the current lane instead of at the `terminal_start` position, as indicated by the dashed red line in the image above.
+
 ## Parameters
 
 ### Essential lane change parameters