doc: update gcp repo README
doc: update gcp repo README

Change-Id: Ifc9e507ec02023b40a98b3404f21e50336cd7f5d
Reviewed-on: https://upspin-review.googlesource.com/9304
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp