This file can be found in the folder 'minimal_overlay/rootfs'. You can use this folder to put your own content (files and folders) which will be visible in the root folder of your 'Minimal Linux Live' system, just like this file is visible. The files/folders will override the content in the root folder, so be careful what you put there because you may end up with broken system. If the boot media is writeable, then all changes on the root file system are persisted automatically, so be very careful what you do when you use the overlay support. Check the '.config' file in the source folder for more details on the overlay options and how to use them.