[release-branch.go1.12-security] doc: document Go 1.12.16

Change-Id: Ib8ac9bf5020d9ab126a8069378978d7dce3509dc
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/648870Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/devel/release.html

@@ -139,6 +139,13 @@ the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.15+label%3
 1.12.15 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.16 (released 2020/01/28) includes two security fixes to
+the <code>crypto/x509</code> package. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.16+label%3ACherryPickApproved">Go
+1.12.16 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>