proposal: add natural XML proposal
proposal: add natural XML proposal

See golang/go#13504

Change-Id: Ie9877b10ae3eed8ad5e5763d35e48d94c6f8f584
Reviewed-on: https://go-review.googlesource.com/30364
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal