[release-branch.go1.12] doc: document Go 1.12.9

Change-Id: I88b7e085fc70f9c021788d364099f5bc6b705ba8
Reviewed-on: https://go-review.googlesource.com/c/go/+/190438Reviewed-by: Filippo Valsorda <filippo@golang.org>
(cherry picked from commit 0212f041)
Reviewed-on: https://go-review.googlesource.com/c/go/+/190406Reviewed-by: Alexander Rakoczy <alex@golang.org>

doc/devel/release.html

@@ -91,6 +91,13 @@ 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>
 
+<p>
+go1.12.9 (released 2019/08/15) includes fixes to the linker,
+and the <code>os</code> and <code>math/big</code> packages.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.9+label%3ACherryPickApproved">Go
+1.12.9 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>