Add copyright in README

README.md

@@ -43,3 +43,20 @@ Generate bar chart with commits by version (tag):
 ## Demo
 `gitchart.py` is used to build statistics for WeeChat: http://weechat.org/dev/stats/
+
+## Copyright
+
+Copyright (C) 2013-2015 Sébastien Helleu <flashcode@flashtux.org>
+
+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/>.