doc: document Go 1.6.1 and Go 1.5.4

Change-Id: Icb9e947a43fb87fbfe0655b09e0d7e8f61825aeb
Reviewed-on: https://go-review.googlesource.com/21825Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/21826Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/devel/release.html

@@ -37,6 +37,14 @@ Go 1.6 is a major release of Go.
 Read the <a href="/doc/go1.6">Go 1.6 Release Notes</a> for more information.
 </p>
 
+<h3 id="go1.6.minor">Minor revisions</h3>
+
+<p>
+go1.6.1 (released 2016/04/12) includes two security fixes.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.1">Go
+1.6.1 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.5">go1.5 (released 2015/08/19)</h2>
 
 <p>
@@ -69,6 +77,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.5.3">Go 1
 and the <a href="https://golang.org/s/go153announce">release announcement</a> for details.
 </p>
 
+<p>
+go1.5.4 (released 2016/04/12) includes two security fixes.
+It contains the same fixes as Go 1.6.1 and was released at the same time.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.1">Go
+1.6.1 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.4">go1.4 (released 2014/12/10)</h2>
 
 <p>