dwm.1: add keybinding for spawning dmenu

dwm.1

@@ -59,6 +59,11 @@ click on a tag label adds/removes that tag to/from the focused window.
 Start
 .BR st(1).
 .TP
+.B Mod1\-p
+Spawn
+.BR dmenu(1)
+for launching other programs.
+.TP
 .B Mod1\-,
 Focus previous screen, if any.
 .TP