Add licence file

LICENCE

+Copyright (c) 2020 Nexedi SA and Contributors.
+
+This program is free software: you can Use, Study, Modify and Redistribute
+it under the terms of the GNU General Public License version 3, or (at your
+option) any later version, as published by the Free Software Foundation.
+
+You can also Link and Combine this program with other software covered by
+the terms of any of the Free Software licenses or any of the Open Source
+Initiative approved licenses and Convey the resulting work. Corresponding
+source of such a combination shall include the source code for all other
+software used.
+
+This program is distributed WITHOUT ANY WARRANTY; without even the implied
+warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+See COPYING file for full licensing terms.
+See https://www.nexedi.com/licensing for rationale and options.
+