docs: add example systemd service file

This is an example systemd service file for lnd designed to run
alongside a bitcoind service. This file is loosely based on
https://github.com/bitcoin/bitcoin/blob/master/contrib/init/bitcoind.service,
https://gist.github.com/Stadicus/f0c6db4fa6cf787f19d9d3444b91633f,
and my own experience managing lnd with systemd.

contrib/init/README.md

@@ -0,0 +1,27 @@
+# Init
+
+Sample configuration files for:
+
+```
+systemd: lnd.service
+```
+
+## systemd
+
+Add the example `lnd.service` file to `/etc/systemd/system/` and modify it according to your system and user configuration. Use the following commands to interact with the service:
+
+```bash
+# Enable lnd to automatically start on system boot
+systemctl enable lnd
+
+# Start lnd
+systemctl start lnd
+
+# Restart lnd
+systemctl restart lnd
+
+# Stop lnd
+systemctl stop lnd
+```
+
+Systemd will attempt to restart lnd automatically if it crashes or otherwise stops unexpectedly.