Add copyright

COPYRIGHT.txt

@@ -0,0 +1,15 @@
+All code in this repository is Copyright 2018 by Angus McLeod.
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 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 General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program as the file LICENSE.txt; if not, please see
+http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt.