Add LICENSE

LICENSE

+Copyright (C)  2018  Orestis Malaspinas.
+
+
+ Permission is granted to copy, distribute and/or modify this document
+ under the terms of the GNU Free Documentation License, Version 1.3
+ or any later version published by the Free Software Foundation;
+ with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
+ A copy of the license can be downloaded from:
+ https://www.gnu.org/licenses/fdl.html.