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

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+# 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.

contrib/init/lnd.service

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+# A sample systemd service file for lnd running with a bitcoind service.
+
+[Unit]
+Description=Lightning Network Daemon
+
+# Make sure lnd starts after bitcoind is ready
+Requires=bitcoind.service
+After=bitcoind.service
+
+[Service]
+ExecStart=/usr/local/bin/lnd
+ExecStop=/usr/local/bin/lncli stop
+PIDFile=/home/bitcoin/.lnd/lnd.pid
+
+# Replace these with the user:group that will run lnd
+User=bitcoin
+Group=bitcoin
+
+# Try restarting lnd if it stops due to a failure
+Restart=on-failure
+RestartSec=60
+
+# Type=notify is required for lnd to notify systemd when it is ready
+Type=notify
+
+# An extended timeout period is needed to allow for database compaction
+# and other time intensive operations during startup. We also extend the
+# stop timeout to ensure graceful shutdowns of lnd.
+TimeoutStartSec=1200
+TimeoutStopSec=3600
+
+# Hardening Measures
+####################
+
+# Mount /usr, /boot/ and /etc read-only for the process.
+ProtectSystem=full
+
+# Disallow the process and all of its children to gain
+# new privileges through execve().
+NoNewPrivileges=true
+
+# Use a new /dev namespace only populated with API pseudo devices
+# such as /dev/null, /dev/zero and /dev/random.
+PrivateDevices=true
+
+# Deny the creation of writable and executable memory mappings.
+MemoryDenyWriteExecute=true
+
+[Install]
+WantedBy=multi-user.target

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

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+# 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