design/11970: design doc for decentralized GC coordination
design/11970: design doc for decentralized GC coordination

Updates golang/go#11970.

Change-Id: Ief40300c9c0da583d940823a2290fbe5d5ad02bf
Reviewed-on: https://go-review.googlesource.com/16293
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal