[release-branch.go1.12] doc: document Go 1.12.15

Change-Id: I3b2c26d4818ca28a71a7fd6927a0c39c9253f06f
Reviewed-on: https://go-review.googlesource.com/c/go/+/214079Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/devel/release.html

@@ -132,6 +132,13 @@ the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.14+label%3
 1.12.14 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.15 (released 2020/01/09) includes fixes to the runtime and
+the <code>net/http</code> package. See
+the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.15+label%3ACherryPickApproved">Go
+1.12.15 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>