doc: 2016 is the Year of the Gopher.

Change-Id: I85caba8c743dcd82954de75df947053b3d206bdc
Reviewed-on: https://go-review.googlesource.com/18117Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

doc/contribute.html

@@ -674,7 +674,7 @@ These files will be periodically updated based on the commit logs.
 <p>Code that you contribute should use the standard copyright header:</p>
 
 <pre>
-// Copyright 2015 The Go Authors. All rights reserved.
+// Copyright 2016 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 </pre>