Add a SystemD template service for owntone (issue #1353)

Create a SystemD template and document how to use it for multiple instances of owntone

README.md

@@ -52,6 +52,7 @@ please see the [INSTALL.md](INSTALL.md) file.
 - [Spotify](#spotify)
 - [LastFM](#lastfm)
 - [MPD clients](#mpd-clients)
+- [Running Multiple Instances](#running-multiple-instances)
 - [References](#references)
 
 
@@ -569,6 +570,30 @@ The following table shows what is working for a selection of MPD clients:
 | [mpc](http://www.musicpd.org/clients/mpc/)    | CLI    | Working commands: mpc, add, crop, current, del (ranges are not yet supported), play, next, prev (behaves like cdprev), pause, toggle, cdprev, seek, clear, outputs, enable, disable, playlist, ls, load, volume, repeat, random, single, search, find, list, update (initiates an init-rescan, the path argument is not supported)   |
 | [ympd](http://www.ympd.org/)                  | Web    | Everything except "add stream" should work |
 
+## Running Multiple Instances
+
+To run multiple instances of owntone on a server, you should copy
+`/etc/owntone.conf` to `/etc/owntone-zone.conf` (for each `zone`) and
+modify the following to be unique across all instances:
+
+* the three port settings (`general` -> `websocket_port`,
+  `library` -> `port`, and `mpd` -> `port`)
+
+* the database paths (`general` -> `db_path`, `db_backup_path`, and `db_cache_path`)
+
+* the service name (`library` -> `name`).
+
+* you probably also want to disable local output (set `audio` -> `type =
+  "disabled"`).
+
+Then run `owntone -c /etc/owntone-zone.conf` to run owntone with the new
+zone configuration.
+
+Owntone has a `systemd` template which lets you run this automatically
+on systems that use systemd.  You can start or enable the service for
+a `zone` by `sudo systemctl start owntone@zone` and check that it is
+running with `sudo systemctl status owntone@zone`.  Use `sudo
+systemctl enable ownton@zone` to get the service to start on reboot.
 
 ## References