add license information

contributors.md

@@ -1,3 +1,21 @@
+<!-- 
+beNNch - Unified execution, collection, analysis and
+comparison of neural network simulation benchmarks.
+Copyright (C) 2021 Forschungszentrum Juelich GmbH, INM-6
+
+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 <https://www.gnu.org/licenses/>.
+
+SPDX-License-Identifier: GPL-3.0-or-later
+-->
+
 # Authors and contributors
 
 The following people have contributed code and/or ideas to the current version of beNNch.