[release-branch.go1.8] website: mention go1.8 in project page

Fixes #19253

Change-Id: Ia473f51bfe4cf42cf64938993a81d9b1dbc2594d
Reviewed-on: https://go-review.googlesource.com/37433Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/37398

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/go1.8">Go 1.8</a> <small>(February 2017)</small></li>
 	<li><a href="/doc/go1.7">Go 1.7</a> <small>(August 2016)</small></li>
 	<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>