[release-branch.go1.6] doc: add link to 1.6 release notes on golang.org/project page

Fixes #14378.

Change-Id: I680b523c01576e42f1dbda9131d1f6bb99b3668b
Reviewed-on: https://go-review.googlesource.com/20138Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/20139Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/contrib.html

@@ -34,6 +34,7 @@ We encourage all Go users to subscribe to
 <p>A <a href="/doc/devel/release.html">summary</a> of the changes between Go releases. Notes for the major releases:</p>
 
 <ul>
+	<li><a href="/doc/go1.6">Go 1.6</a> <small>(February 2016)</small></li>
 	<li><a href="/doc/go1.5">Go 1.5</a> <small>(August 2015)</small></li>
 	<li><a href="/doc/go1.4">Go 1.4</a> <small>(December 2014)</small></li>
 	<li><a href="/doc/go1.3">Go 1.3</a> <small>(June 2014)</small></li>