[gthumb] readme: clarified license version

[Paolo Bacchilega <paobac src gnome org>]

commit 14f43a6486724cfc6b08b2da54f08c6f89fe1cf0
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Mon Nov 11 17:34:21 2019 +0100

    readme: clarified license version

 README | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/README b/README
index dd6f6dbe..305e409f 100644
--- a/README
+++ b/README
@@ -59,7 +59,8 @@ Licensing
 =========
 
   This program is released under the terms of the GNU General Public
-  License (GNU GPL).
+  License (GNU GPL), either version 2 of the License, or (at your option) any 
+  later version.
 
   You can find a copy of the license in the file COPYING.