service definition added

README.md

@@ -13,8 +13,27 @@ As the `pi` user in its home directory:
 * `python3 -m venv darkmute-venv`
 * `. darkmute-venv/bin/activate`
 * `sudo python3 -m pip install DarkMute`
+* Test it: `darkmute`
 
-To run this as a service, create a systemd service definition. Instructions pending.
+Brightness thresholds are hard-coded for now, to change them see Developer Setup section.
+
+To run this as a service, create a systemd service definition:
+
+/etc/systemd/system/darkmute.service
+
+```
+[Unit]
+Description=Dark Mute
+After=snapclient.service
+
+[Service]
+ExecStart=/home/pi/darkmute-venv/bin/darkmute
+Restart=Always
+User=pi
+
+[Install]
+WantedBy=multi-user.target
+```
 
 # Developer Setup