Add license to README

Author: Birgit Schachler

README.rst

@@ -55,3 +55,21 @@ Matplotlib can be installed using pip3 but some Linux users reported that it is
 
 http://matplotlib.org/users/installing.html
 
+
+License
+============
+
+Copyright (C) 2017 oemof developing group
+
+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/.