design: add draft-vulndb.md
design: add draft-vulndb.md

Change-Id: Idc4853f911c4edaaa8e618721af337c28b1dfccc
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/303170
Reviewed-by: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>

golang/proposal,golang/proposal