[release-branch.go1.12] doc: document Go 1.12.1

Change-Id: I6d3a615c5f72e9aa29d23e127af98d6e836da173
Reviewed-on: https://go-review.googlesource.com/c/go/+/167699Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit f832a97e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167702Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/devel/release.html

@@ -30,6 +30,14 @@ Go 1.12 is a major release of Go.
 Read the <a href="/doc/go1.12">Go 1.12 Release Notes</a> for more information.
 </p>
 
+<p>
+go1.12.1 (released 2019/03/14) includes fixes to cgo, the compiler, the go
+command, and the <code>fmt</code>, <code>net/smtp</code>, <code>os</code>,
+<code>path/filepath</code>, <code>sync</code>, and <code>text/template</code>
+packages. See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.1">Go
+1.12.1 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>