added documentation for preserving files

Author: hyeenaa

_docs/sysadmin/installation/system_customization.md

@@ -372,6 +372,21 @@ You can customize the login screen with markdown. By default, `# Login` is rende
 
 Refer to [this](/student/communication/markdown) to learn more about markdown.
 
+## Preserving your changes on system re-installation
+
+you can prevent your changes from being overwritten from a system re-install by entering paths into a JSON file.
+
+the JSON file will be generated at `/usr/local/submitty/config/preserve_files_list.json`
+
+the format will be as follows:  
+
+  ```
+  [  
+    "/path/to/file",  
+    "/more/files/,  
+    ...  
+  ]
+```
 
 
 ## Changing the multi-processing module of Apache2, Enabling HTTP2