main: revise warnings

main_windows.go

@@ -27,6 +27,8 @@ func main() {
 	}
 	interfaceName := os.Args[1]
 
+	fmt.Fprintln(os.Stderr, "Warning: this is a test program for Windows, mainly used for debugging this Go package. For a real WireGuard for Windows client, the repo you want is <https://git.zx2c4.com/wireguard-windows/>, which includes this code as a module.")
+
 	logger := device.NewLogger(
 		device.LogLevelDebug,
 		fmt.Sprintf("(%s) ", interfaceName),