doc: document go1.9.1 and go1.8.4

Change-Id: Ib42fabc6829b6033373c0378713733f88e73e73d
Reviewed-on: https://go-review.googlesource.com/68230Reviewed-by: Russ Cox <rsc@golang.org>

doc/devel/release.html

@@ -30,6 +30,12 @@ Go 1.9 is a major release of Go.
 Read the <a href="/doc/go1.9">Go 1.9 Release Notes</a> for more information.
 </p>
 
+<p>
+go1.9.1 (released 2017/10/04) includes two security fixes.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.9.1">Go
+1.9.1 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.8">go1.8 (released 2017/02/16)</h2>
 
 <p>
@@ -63,6 +69,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.8.3">Go
 1.8.3 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.8.4 (released 2017/10/04) includes two security fixes.
+It contains the same fixes as Go 1.9.1 and was released at the same time.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.8.4">Go
+1.8.4 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.7">go1.7 (released 2016/08/15)</h2>
 
 <p>