design: add 25719-go15vendor.md
design: add 25719-go15vendor.md

This is a port of the Google Doc at
https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/edit.

Fixes golang/go#25719.

Change-Id: Ifa73929764d819f8e8caf41277c74e9d4bbeee21
Reviewed-on: https://go-review.googlesource.com/116177
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal