doc: document Go 1.3

This is a clone of 101370043, which I accidentally applied to the
release branch first.
No big deal, it needed to be applied there anyway.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/108090043

doc/devel/release.html

@@ -13,6 +13,13 @@ hg pull
 hg update <i>tag</i>
 </pre>
 
+<h2 id="go1.3">go1.3 (released 2014/06/18)</h2>
+
+<p>
+Go 1.3 is a major release of Go.
+Read the <a href="/doc/go1.3">Go 1.3 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.2">go1.2 (released 2013/12/01)</h2>
 
 <p>