doc: document Go 1.13

Change-Id: Icf7e1dab82aa48cc693eb4da8a564dff23312741
Reviewed-on: https://go-review.googlesource.com/c/go/+/192745
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/devel/release.html

@@ -23,6 +23,13 @@ in supported releases as needed by issuing minor revisions
 (for example, Go 1.6.1, Go 1.6.2, and so on).
 </p>
 
+<h2 id="go1.13">go1.13 (released 2019/09/03)</h2>
+
+<p>
+Go 1.13 is a major release of Go.
+Read the <a href="/doc/go1.13">Go 1.13 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.12">go1.12 (released 2019/02/25)</h2>
 
 <p>