README: Clarify license notice

Note that we permit redistribution and modification under the terms of
future GPL versions, and state the exclusion of warranty.  This is in
accordance with the license notices at the top of our source files.

README

@@ -79,11 +79,18 @@ That's it!  If you have any problems or questions, feel free to send an
 email to <help@nagios-plugins.org>.
 
 
-License Stuff
--------------
+License Notice
+--------------
 
-This software is released under the GNU General Public License, Version 3,
-with the additional exemption that compiling, linking and/or using OpenSSL
-is allowed.
+You can redistribute and/or modify this software 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; with the
+additional exemption that compiling, linking, and/or using OpenSSL is
+allowed.
 
-See the `COPYING` file for the complete GPLv3 text.
+This software 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 `COPYING` file for the complete text of the GNU General Public
+License, version 3.