added comment about systemd service

.config/mpd/mpd.conf

@@ -1,3 +1,9 @@
+# since some paths are defined in pam_environment,
+# which is read after user login,
+# enable mpd service by
+# systemctl enable --user mpd
+# after disabling systemwide (sudo systemctl disable mpd)
+
 port                "6600"
 
 log_file            "~/.local/share/mpd/log"