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.

docs/release-notes/release-notes-0.15.0.md

@@ -0,0 +1,11 @@
+# Release Notes
+
+## Misc
+
+* [An example systemd service file](https://github.com/lightningnetwork/lnd/pull/6033)
+  for running lnd alongside a bitcoind service is now provided in
+  `contrib/init/lnd.service`.
+
+# Contributors (Alphabetical Order)
+
+* Daniel McNally