doc: add go1.11 to contrib.html

Missing from https://golang.org/project

Change-Id: I6cb769ae861a81f0264bae624b5fe8d70aa92497
Reviewed-on: https://go-review.googlesource.com/138355Reviewed-by: Dave Cheney <dave@cheney.net>

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/go1.11">Go 1.11</a> <small>(August 2018)</small></li>
 	<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>