add `-v` flag to the manpage

Thanks @Humm42

dwl.1

@@ -6,6 +6,7 @@
 .Nd dwm for Wayland
 .Sh SYNOPSIS
 .Nm
+.Op Fl v
 .Op Fl s Ar command
 .Sh DESCRIPTION
 .Nm
@@ -15,6 +16,12 @@ It is intended to fill the same space in the Wayland world that
 does for X11.
 .Pp
 When given the
+.Fl v
+option,
+.Nm
+writes its name and version to standard error and exits unsuccessfully.
+.Pp
+When given the
 .Fl s
 option,
 .Nm

dwl.c

@@ -2536,5 +2536,5 @@ main(int argc, char *argv[])
 	return EXIT_SUCCESS;
 
 usage:
-	die("Usage: %s [-s startup command]", argv[0]);
+	die("Usage: %s [-v] [-s startup command]", argv[0]);
 }