Source the config or catkin build fails

README.md

@@ -39,6 +39,7 @@ instructions further below in this document. Please note that the setup uses vnc
         - `git clone https://git-st.inf.tu-dresden.de/ceti/ros/sample_applications.git` **or** `git clone git@git-st.inf.tu-dresden.de:ceti/ros/sample_applications.git`
         - change into the workspace main directory `cd ..`
 - install ROS package dependencies `rosdep install -i --from-paths .`
+- source the config: `source /opt/ros/melodic/setup.bash`
 - build the workspace `catkin build`
 - source the config: depending on your shell
     - `source devel/setup.bash` (default)