Add SPDX

src/mDash.h

@@ -1,5 +1,20 @@
 // Copyright (c) 2019-2022 Cesanta Software Limited
 // All rights reserved
+//
+// This software is dual-licensed: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License version 2 as
+// published by the Free Software Foundation. For the terms of this
+// license, see http://www.gnu.org/licenses/
+//
+// You are free to use this software under the terms of the GNU General
+// Public License, but WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU General Public License for more details.
+//
+// Alternatively, you can license this software under a commercial
+// license, as set out in https://www.mongoose.ws/licensing/
+//
+// SPDX-License-Identifier: GPL-2.0 or commercial
 
 #pragma once