docs: update security issues contact (#1136)
docs: update security issues contact (#1136)

Use team@coredns.io for reporting sec. vulnerablities. Currenttly this
has john@coredns.io and miek@coredns.io
chrisohaver/coredns,ii/coredns,miekg/coredns,jremond/coredns,johnbelamaric/coredns,ii/coredns,jremond/coredns,miekg/coredns,johnbelamaric/coredns,yongtang/coredns,coredns/coredns