doc: update CONTRIBUTING.md (#2530)
doc: update CONTRIBUTING.md (#2530)

* doc: update CONTRIBUTING.md

Remove bunch of old/stale text, but add a section (and clarify) using
issues to describe a future PR and that those need to be small (if you
want it merged faster)

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

* Update CONTRIBUTING.md

Co-Authored-By: miekg <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

ii/coredns,coredns/coredns,johnbelamaric/coredns,miekg/coredns,ii/coredns,johnbelamaric/coredns,chrisohaver/coredns,miekg/coredns,yongtang/coredns