Changed gpl2->gpl3 in readme because of gpl3 parts in the QGIS sources

git-svn-id: http://svn.osgeo.org/qgis/trunk@15321 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: mhugent

README

@@ -17,7 +17,7 @@ daily GIS data viewing and editing needs. QGIS supports a number of raster
 and vector data formats, with new support easily added using the plugin
 architecture.
 
-QGIS is released under the GNU Public License (GPL) Version 2. Developing
+QGIS is released under the GNU Public License (GPL) Version 3. Developing
 QGIS under this license means that you can (if you want to) inspect and
 modify the source code and guarantees that you, our happy user will always
 have access to a GIS program that is free of cost and can be freely