[release-branch.go1.12-security] doc: document Go 1.12.10

Change-Id: If694ce529393b8ae9c6c55270665efc3a108a3b2
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/558778Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/558784

doc/devel/release.html

@@ -98,6 +98,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.9+labe
 1.12.9 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.10 (released 2019/09/25) includes security fixes to the
+<code>net/http</code> and <code>net/textproto</code> packages.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.10">Go
+1.12.10 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>