added Button{4,5} description to dwm.1

dwm.1

@@ -44,6 +44,12 @@ label toggles between tiling and floating mode.
 .B Button3
 click on a tag label adds/removes all windows with that tag to/from the view.
 .TP
+.B Button4
+click on the mode label increases the number of windows in the master area (tiling mode only).
+.TP
+.B Button5
+click on the mode label decreases the number of windows in the master area (tiling mode only).
+.TP
 .B Mod1-Button1
 click on a tag label applies that tag to the focused window.
 .TP