add systemd service file

contrib/systemd/ashuffle.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Shuffle service for mpd
+Requires=mpd.service
+After=mpd.service
+
+[Service]
+ExecStart=/usr/bin/ashuffle --tweak play-on-startup=no
+
+[Install]
+WantedBy=default.target

contrib/systemd/readme.md

@@ -0,0 +1,27 @@
+If your distributions package provides the service file, you can start the ashuffle service with:
+
+    $ systemctl --user start ashuffle.service
+
+use systemctl enable to autostart ashuffle on every login:
+
+    $ systemctl --user enable ashuffle.service
+
+
+If your installed package doesn't provide the service file, just copy it into your systemd user directory before using the above commands:
+
+    ~/.config/systemd/user/ashuffle.service
+
+
+
+To start the ashuffle service with parameters, use the systemctl edit command:
+
+    systemctl --user edit ashuffle.service
+
+this creates and opens the file ~/.config/systemd/user/ashuffle.service.d/override.conf, insert something like this:
+```
+    [Service]
+    ExecStart=
+    ExecStart=/usr/bin/ashuffle --only 10 --tweak play-on-startup=no
+```
+
+Note how ExecStart must be cleared before being re-assigned.