[release-branch.go1.13] doc: document Go 1.13.5

Change-Id: I289d13ff0a01466d93ebc555eaa81273d4297eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/209841


Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit ebfe0574)
Reviewed-on: https://go-review.googlesource.com/c/go/+/209843
Run-TryBot: Alexander Rakoczy <alex@golang.org>