doc: Update copyright years; add visible copyright statement.

* doc/guix.texi (YEARS): New variable.
  Use it for all copyright statements.
  (Top): Add a copyright statement and license header here.

doc/guix.texi

@@ -8,6 +8,7 @@
 @c %**end of header
 
 @include version.texi
+@set YEARS 2012, 2013
 
 @dircategory Package management
 @direntry
@@ -28,7 +29,7 @@
 Edition @value{EDITION} @*
 @value{UPDATED} @*
 
-Copyright @copyright{} 2012 Ludovic Court@`es
+Copyright @copyright{} @value{YEARS} Ludovic Court@`es
 
 @quotation
 Permission is granted to copy, distribute and/or modify this document
@@ -43,7 +44,7 @@ Documentation License''.
 @copying
 This manual documents GNU Guix version @value{VERSION}.
 
-Copyright (C) 2012, 2013 Ludovic Courtès
+Copyright @copyright{} @value{YEARS} Ludovic Courtès
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -62,6 +63,17 @@ Documentation License.''
 This document describes GNU Guix version @value{VERSION}, a functional
 package management tool written for the GNU system.
 
+@quotation
+Copyright @copyright{} @value{YEARS} Ludovic Courtès
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with no
+Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
+copy of the license is included in the section entitled ``GNU Free
+Documentation License.''
+@end quotation
+
 @menu
 * Introduction::                What is Guix about?
 * Installation::                Installing Guix.