Update ubuntu-persistent-installation.md

docs/ubuntu-persistent-installation.md

@@ -30,6 +30,16 @@ Skip the welcoming window.
 
 Don't install updates if Ubuntu asks to. The scripts will.
 
+## OS customization
+
+You can overwrite the files of the folder `system` by setting an environment variable named `OVERWRITE` with the format : `Destination folder;Git URL`.
+
+For example to write the content of the repository [github.com/xpetit/custom](https://github.com/xpetit/custom) in the system folder :
+
+```shell
+export OVERWRITE='.;https://github.com/xpetit/custom.git'
+```
+
 ## OS configuration
 
 Run a terminal and type these commands :