Added note to change parameter for urxvt users

It took me a while to realize the error, so this might spare some time of other urxvt users.

.scripts/i3cmds/ddspawn

@@ -13,5 +13,6 @@ then
 	i3 "[instance=\"dropdown_$1\"] scratchpad show; [instance=\"dropdown_$1\"] move position center"
 else
 	echo "Window not detected... spawning."
+	# urxvt users have to specify the name via -name instead of -n
 	i3 "exec --no-startup-id $TERMINAL -n dropdown_$1 $(echo "$@" | cut -d ' ' -f2-) -e $1"
 fi