design: add 25530-notary.md

See https://golang.org/design/25530-notary.

For golang/go#25530.

Change-Id: I1b4add8fe1c2f6911e925bafab99eb7418aa67b4
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/165018
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: rsc