Launch-In-SITL.md

@@ -6,7 +6,13 @@ cd ardupilot_ws
 catkin init
 cd src
 ```
-Clone the reprository into src
+**Note:** if you have issue "catkin not found":
+```shell
+cd ~
+echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc
+source ~/.bashrc
+```
+**Clone** the reprository into src
 
 Build the node with catkin_make:
 ```shell