[docs] Add note to source setup.bash in ROS pkgs

docs/airsim_tutorial_pkgs.md

@@ -5,11 +5,22 @@ See [airsim_ros_pkgs](https://github.com/microsoft/AirSim/blob/master/ros/src/ai
 
 
 ## Setup
+
+Make sure that the [airsim_ros_pkgs Setup](airsim_ros_pkgs.md) has been completed and the prerequisites installed.
+
 ```shell
 $ cd PATH_TO/AirSim/ros
 $ catkin build airsim_tutorial_pkgs
 ```
 
+If your default GCC isn't 8 or greater (check using `gcc --version`), then compilation will fail. In that case, use `gcc-8` explicitly as follows-
+
+```shell
+catkin build airsim_tutorial_pkgs -DCMAKE_C_COMPILER=gcc-8 -DCMAKE_CXX_COMPILER=g++-8
+```
+
+Note: For running examples, and also whenever a new terminal is opened, sourcing the `setup.bash` file is necessary. If you're using the ROS wrapper frequently, it might be helpful to add the `source PATH_TO/AirSim/ros/devel/setup.bash` to your `~/.profile` or `~/.bashrc` to avoid the need to run the command every time a new terminal is opened
+
 ## Examples
 
 ### Single drone with monocular and depth cameras, and lidar
@@ -24,6 +35,7 @@ $ catkin build airsim_tutorial_pkgs
  $ roslaunch airsim_ros_pkgs airsim_node.launch;
 
  # in a new pane / terminal
+ $ source PATH_TO/AirSim/ros/devel/setup.bash
  $ roslaunch airsim_tutorial_pkgs front_stereo_and_center_mono.launch
  ```
  The above would start rviz with tf's, registered RGBD cloud using [depth_image_proc](https://wiki.ros.org/depth_image_proc) using the [`depth_to_pointcloud` launch file](https://github.com/microsoft/AirSim/master/ros/src/airsim_tutorial_pkgs/launch/front_stereo_and_center_mono/depth_to_pointcloud.launch), and the lidar point cloud.