add info on "on-click-middle" option

man/waybar-custom.5.scd

@@ -59,6 +59,10 @@ Addressed by *custom/<name>*
 	typeof: string ++
 	Command to execute when clicked on the module.
 
+*on-click-middle*: ++
+        typeof: string ++
+        Command to execute when middle-clicked on the module using mousewheel.
+
 *on-click-right*: ++
 	typeof: string ++
 	Command to execute when you right clicked on the module.
@@ -137,6 +141,7 @@ $text\\n$tooltip\\n$class*
 	"exec": "mpc current",
 	"exec-if": "pgrep mpd",
 	"on-click": "mpc toggle",
+	"on-click-middle": "playerctl play-pause",
 	"on-click-right": "sonata"
 }
 ```