add docs for custom x11 config in readme

Author: hertg

README.md

@@ -150,6 +150,30 @@ hooks:
   egpu: /home/michael/tmp/egpu.sh
 ```
 
+### Custom X11 config
+
+Since version `0.20.0`, a custom x11 config template can be configured by creating a file at `/usr/share/egpu-switcher/x11-template.conf`. The default template is shown below, you can use it as a starting point.
+
+```
+# autogenerated by egpu-switcher
+{{if .Modesetting}}
+Section "Module"
+    Load           "modesetting"
+EndSection
+{{end}}
+
+Section "Device"
+    Identifier     "{{.Id}}"
+    Driver         "{{.Driver}}"
+    BusID          "{{.Bus}}"
+    Option         "AllowEmptyInitialConfiguration"
+    Option         "AllowExternalGpus" "True"
+EndSection
+```
+
+> [!TIP]
+> For safety, and to stop egpu-switcher from printing warnings, the file should be owned by `root:root` and have a mode of `644`.
+
 ---
 
 ## Troubleshooting