Missing CONTRIBUTING.md file

[aojea]

All K8s subrepositories should have a CONTRIBUTING.md file, which at the minimum should point to https://github.com/kubernetes/community/blob/master/contributors/guide/README.md. Care should be taken that all information is in sync with the contributor guide.

Subrepositories may also have contributing guidelines specific to that repository. They should be explicitly documented and explained in the CONTRIBUTING.md

Ref: kubernetes/community#1832