doc: document Go 1.12.8 and Go 1.11.13

Change-Id: I0daab6cd347e1fc0066e516f02c33f1b63e3f1a3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/526992Reviewed-by: Filippo Valsorda <valsorda@google.com>
(cherry picked from commit 305f6dc3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/190437Reviewed-by: Filippo Valsorda <filippo@golang.org>

doc/devel/release.html

@@ -84,6 +84,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.7">Go
 1.12.7 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.8 (released 2019/08/13) includes security fixes to the
+<code>net/http</code> and <code>net/url</code> packages.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.8">Go
+1.12.8 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>
@@ -181,6 +188,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.11.12">Go
 1.11.12 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.11.13 (released 2019/08/13) includes security fixes to the
+<code>net/http</code> and <code>net/url</code> packages.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.11.13">Go
+1.11.13 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.10">go1.10 (released 2018/02/16)</h2>
 
 <p>