CONTRIBUTORS: update for the Go 1.15 release

CONTRIBUTORS: update for the Go 1.15 release

This update was created using the updatecontrib command:

	go get golang.org/x/build/cmd/updatecontrib
	cd gotip
	updatecontrib

With manual changes based on publicly available information
to canonicalize letter case and formatting for a few names.

For #12042.

Change-Id: I66dc5ee28d9a64bc9d150e72d136d8f71e50373b
Reviewed-on: https://go-review.googlesource.com/c/go/+/247767
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>