more elaborate setup steps required

[Satish1989221]

1. after cloning ardupilot_sitl_gazebo_plugin in /home/catkin_ws/src, I get a fatal error: mav_msgs/CommandMotorSpeed.h: No such file or directory
   #include <mav_msgs/CommandMotorSpeed.h>
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   I am not sure if mav_msgs from ETHZ's repository needs to pass its build first before setting this up.

2. Are the folders inside this repository ros packages?

3. Should this work for ros melodic?

4. The steps located here suggest that ardupilot_sitl_gazebo_plugin must be compiled inside /opt/ros/melodic/src is that right?