improve README

README.md

@@ -1,4 +1,23 @@
 # mpdyn
 
 Maintain a dynamic mpd playlist.
-mpdyn enables the consume mode and add songs from your library automatically 
+mpdyn enables the consume mode and add songs from your library automatically 
+
+Every 20s, mpdyn will check the playlist. If it's made of less than 10 tracks, it will then add as many random tracks as needed to refill it.
+
+It can be started manually or through a systemd user service.
+
+### Systemd
+
+Here is a service example:
+
+    # cat ~/.config/systemd/user/mpdyn.service
+    [Unit]
+    Description=mpdyn
+
+    [Service]
+    ExecStart=/home/taziden/.cargo/bin/mpdyn
+
+    [Install]
+    WantedBy=default.target
+