Update README.md

Add more information about unit testing to the "Development" section

Author: jamesps-ebi

README.md

@@ -128,4 +128,6 @@ This module doesn't perform any comprehensive validation for the generated netpl
 
 ## Development
 
-Please submit your contributions and issues to GitHub: https://github.com/jps-help/puppet-netplan
+Please submit your contributions and issues to GitHub: https://github.com/jps-help/puppet-netplan
+
+To make life easier for myself, I'm using (ghcr.io/voxpupuli/voxbox)[https://github.com/voxpupuli/container-voxbox] to perform unit testing. See their README.md for more information.