Update run instructions according to #21

INSTALL.md

@@ -117,9 +117,15 @@ source .venv/bin/activate
 ```
 and run the photobooth as
 ```bash
-python photobooth
+python -m photobooth
 ```
 
+Alternatively, use the Python binary of the virtual environment to start the photobooth directly without activating the environment first:
+```bash
+.venv/bin/python -m photobooth
+```
+This is useful, e.g., when starting the photobooth from scripts, desktop shortcuts, or when using an autostart mechanism of your window manager.
+
 Change any settings via the "Settings" menu.
 Afterwards, select "Start photobooth" to get started.
 You can trigger the countdown via space bar or an external button.