Update copyright year and add license

plugins/disk/du-2

@@ -5,10 +5,9 @@
 
   du - Plugin to monitor multiple directories size
 
-=head1 AUTHOR
+=head1 AUTHOR AND COPYRIGHT
 
-  Luc Didry <luc AT didry.org>
-  April 2011
+  Copyright 2011-2012 Luc Didry <luc AT didry.org>
 
 =head1 HOWTO CONFIGURE AND USE :
 
@@ -44,6 +43,21 @@
   #%# family=auto
   #%# capabilities=autoconf
 
+=head1 LICENSE
+
+    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
+    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/>.
+
 =cut
 
 use warnings;