doc: document Go 1.12.2

Change-Id: I990c451ff24844b39dee2477cec4caa9db2e8ebb
Reviewed-on: https://go-review.googlesource.com/c/go/+/170883Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/devel/release.html

@@ -40,6 +40,14 @@ packages. See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1
 1.12.1 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.2 (released 2019/04/05) includes fixes to the compiler, the go
+command, the runtime, and the <code>doc</code>, <code>net</code>,
+<code>net/http/httputil</code>, and <code>os</code> packages. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.2">Go
+1.12.2 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>