Relicense under AGPLv3 and update information in readme.txt

advtrains/init.lua

@@ -1,4 +1,24 @@
 
+--[[
+Advanced Trains - Minetest Mod
+
+Copyright (C) 2016-2020  Moritz Blei (orwell96) and contributors
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as
+    published by the Free Software Foundation, either version 3 of the
+    License, or (at your option) any later version.
+
+    This program 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 Affero General Public License for more details.
+
+    You should have received a copy of the GNU Affero General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+]]
+
 local lot = os.clock()
 minetest.log("action", "[advtrains] Loading...")