internal/history: document upcoming Go 1.15.1 and Go 1.14.8 security releases
internal/history: document upcoming Go 1.15.1 and Go 1.14.8 security releases

They have been pre-announced¹.

¹ https://groups.google.com/d/msg/golang-announce/JvvJpgiIfFI/99z802JLAwAJ

Change-Id: I03712ae6af76770970679ea5e53c7f5238cce9f2
Reviewed-on: https://go-review.googlesource.com/c/website/+/251119
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website