doc/contrib.html: add Go 1.10 to release list

Apparently we maintain this list by hand (for example, CL 52351).

Change-Id: I0a0b346cf2b7b547729cb1d0fa1642de447f7bba
Reviewed-on: https://go-review.googlesource.com/78117
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@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/go1.10">Go 1.10</a> <small>(February 2018)</small></li>
 	<li><a href="/doc/go1.9">Go 1.9</a> <small>(August 2017)</small></li>
 	<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>