docs: plugin.md (#2813)
docs: plugin.md (#2813)

Update this file, give Readiness its own section and remove the talk
about the *reverse* plugin as it does not exist in the main tree
anymore.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
chrisohaver/coredns,miekg/coredns,johnbelamaric/coredns,coredns/coredns,ii/coredns,yongtang/coredns,miekg/coredns,ii/coredns,johnbelamaric/coredns