adding GPL license (fixes GeoNode#543)

Author: sbenthall

README.rst

@@ -190,3 +190,22 @@ Directory Structure
     * geoserver-geonode-ext/ - the GeoServer extensions used by the GeoNode.
       Actually, the build script for this project is set up to create a WAR
       that includes those extensions, not just a bundle with the extension.
+
+
+GPL License
+=======
+
+GeoNode is Copyright 2010 OpenPlans.
+
+GeoNode 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.
+
+GeoNode 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 GeoNode.  If not, see <http://www.gnu.org/licenses/>.