doc: document Go 1.13.2 and Go 1.12.11

Change-Id: I73f27924046a0a2493330ddc732d1a2fd3f730a5
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/575981


Reviewed-by: Filippo Valsorda <valsorda@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/575983
Reviewed-on: https://go-review.googlesource.com/c/go/+/201785


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>