doc: add Go 1.1 to release.html

Fixes #5468.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9853051

doc/devel/release.html

@@ -13,6 +13,14 @@ hg pull
 hg update <i>tag</i>
 </pre>
 
+<h2 id="go1.1">go1.1 (released 2013/05/13)</h2>
+
+<p>
+Go 1.1 is a major release of Go.
+Read the <a href="/doc/go1.1.html">Go 1.1 Release Notes</a> for
+more information.
+</p>
+
 <h2 id="go1">go1 (released 2012/03/28)</h2>
 
 <p>