doc: link to Go 1.5 release notes from "project" page

Update issue #12102.

Change-Id: I9cafb284a848cc053bc7e5479c53ebf889b6d4d9
Reviewed-on: https://go-review.googlesource.com/13480Reviewed-by: Rob Pike <r@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/go1.5">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>
 	<li><a href="/doc/go1.2">Go 1.2</a> <small>(December 2013)</small></li>