shell/mpd/bin: notify-song => notify-song.sh (non-interactive script)

shell/mpd/.ncmpcpp/config

@@ -248,7 +248,7 @@ song_columns_list_format = (3f)[8]{NEr:#} (45)[5]{t|f:Title} (25)[13]{a|A:Artist
 ## Note: Custom command that will be executed each
 ## time song changes. Useful for notifications etc.
 ##
-execute_on_song_change = "notify-song"
+execute_on_song_change = "notify-song.sh"
 #playlist_show_mpd_host = no
 playlist_show_remaining_time = yes
 playlist_shorten_total_times = yes

shell/mpd/bin/notify-song.sh

@@ -1,5 +1,9 @@
 #!/usr/bin/env bash
 
+# A non-interactive script that creates notifications (libnotify on Linux,
+# native notifications on MacOS), announcing the currently playing song. Used by
+# ncmpcpp.
+
 if ! pgrep mpd >/dev/null; then
     >&2 echo "mpd isn't running."
     exit 1