added license information

AUTHORS

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+Metagraph was conceived and written by:
+    Mikhail Karasikov
+    Harun Mustafa
+    Christopher Barber
+    Gunnar Raetsch
+    Andre Kahles

COPYRIGHT

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+Metagraph is provided free of charge under the GPLv3 license:
+
+Copyright (c) 2014-2019, Mikhail Karasikov, Harun Mustafa, Andre Kahles, Gunnar Raetsch
+                    (for further author information, please refer to the AUTHORS file)
+All rights reserved.
+
+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 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see http://www.gnu.org/licenses/ .
+
+For the full license text, please refer to the LICENSE file.
+