Add NSID description to README.md. (#1388)
Add NSID description to README.md. (#1388)

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
jremond/coredns,ii/coredns,coredns/coredns,johnbelamaric/coredns,miekg/coredns,ii/coredns,chrisohaver/coredns,johnbelamaric/coredns,jremond/coredns,miekg/coredns,yongtang/coredns