doc: document Go 1.12.12

Change-Id: I832ba5f32d513b586bb0b02371231786b25631e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/201817Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/devel/release.html

@@ -135,6 +135,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.11">Go
 1.12.11 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.12 (released 2019/10/17) includes fixes to the go command,
+runtime, <code>syscall</code> and <code>net</code> packages.
+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>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>