Add COPYING.txt file to clarify coprights and licenses

COPYING.txt

@@ -0,0 +1,30 @@
+The Mesecons Mod for Minetest is
+	Copyright (C) 2011-2016 Mesecons Mod Developer Team and contributors
+
+See the version control system log for information about other authors.
+
+License of source code
+----------------------
+Copyright (C) 2011-2016 Mesecons Mod Developer Team and contributors
+
+This program is free software; you can redistribute the Mesecons Mod and/or
+modify it under the terms of the GNU Lesser General Public License version 3
+published by the Free Software Foundation.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Library General Public License for more details.
+
+You should have received a copy of the GNU Library General Public
+License along with this library; if not, write to the
+Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+Boston, MA  02110-1301, USA.
+
+License of media (textures, sounds and documentation)
+-----------------------------------------------------
+Copyright (C) 2011-2016 Mesecons Mod Developer Team and contributors
+
+All textures, sounds and documentation files are licensed under the
+Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
+http://creativecommons.org/licenses/by-sa/3.0/