Provide license upgrade path.

Thanks to Richard Stallman for suggesting this.

Author: blynn

en/preface.txt

@@ -51,8 +51,10 @@ If I've left you out by mistake, please tell me or just send me a patch!
 
 === License ===
 
-This guide is released under http://www.gnu.org/licenses/gpl-3.0.html[the GNU General Public License version 3]. Naturally, the source is kept in a Git
-repository, and can be obtained by typing:
+This guide is released under http://www.gnu.org/licenses/gpl-3.0.html[the GNU General Public License, version 3] or any later version published by the Free Software Foundation.
+
+Naturally, the source is kept in a Git repository, and can be obtained by
+typing:
 
  $ git clone git://repo.or.cz/gitmagic.git  # Creates "gitmagic" directory.