update GPL blurb to version 3

README.md

@@ -6,7 +6,18 @@ any location, while feeling as close to a real desktop application as possible.
 
 http://tt-rss.org
 
-Licensed under GNU GPL version 2
+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/>.
 
 Copyright (c) 2005 Andrew Dolgov (unless explicitly stated otherwise).