Add systemd service definition for ngrok

I had been running ngrok in screen, but this is better. Now ngrok
comes up automatically on boot.

README.md

@@ -272,6 +272,14 @@ and keep it running.
    sudo systemctl start irremote.service
    ```
 
+### Running ngrok
+
+There is also a systemd service definition for ngrok in `config/systemd`. Follow the same steps
+as above to install and enable it.
+
+The service definition assumes that the ngrok binary is at `/usr/local/bin/ngrok`. Change the service
+definition (both `ConditionPathExists` and `ExecStart`) if you'd like ngrok to live somewhere else.
+
 ## Appendix
 
 ### `Warning: Cannot access device: /dev/lirc0` on kernel 4.19

config/systemd/ngrok.service

@@ -0,0 +1,16 @@
+# /lib/systemd/system/ngrok.service
+# sudo systemctl enable ngrok.service
+[Unit]
+Description=ngrok
+ConditionPathExists=/usr/local/bin/ngrok
+
+[Service]
+Type=simple
+Restart=always
+RestartSec=2
+User=pi
+Group=pi
+ExecStart=/usr/local/bin/ngrok start --all
+
+[Install]
+WantedBy=multi-user.target