[release-branch.go1.6] doc: document go1.6.3

Change-Id: Ib33d7fb529aafcaf8ca7d43b2c9480f30d5c28cc
Reviewed-on: https://go-review.googlesource.com/25011Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/25015Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/devel/release.html

@@ -53,6 +53,14 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.2">Go
 1.6.2 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.6.3 (released 2016/07/17) includes security fixes to the
+<code>net/http/cgi</code> package and <code>net/http</code> package when used in
+a CGI environment. This release also adds support for macOS Sierra.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.3">Go
+1.6.3 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.5">go1.5 (released 2015/08/19)</h2>
 
 <p>