[release-branch.go1.12] doc: document Go 1.12.13

Change-Id: Ic65a74e56320adbd76aeef1cf3b19d7906ffe8fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/204639
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/devel/release.html

@@ -119,6 +119,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.12">Go
 1.12.12 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.13 (released 2019/10/31) fixes an issue on macOS 10.15 Catalina
+where the non-notarized installer and binaries were being
+<a href="https://golang.org/issue/34986">rejected by Gatekeeper</a>.
+Only macOS users who hit this issue need to update.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>