Adding CONTRIBUTING.md for sig-multicluster
Adding CONTRIBUTING.md for sig-multicluster

Inspired from https://github.com/kubernetes/community/blob/master/sig-cli/CONTRIBUTING.md.

Most of the links might be broken at the moment  as sig-federation is renamed to sig-multicluster everywhere.
Will send another PR to rename the parent folder.
kubernetes/community,bgrant0607/community,pwittrock/community,pwittrock/community,kubernetes/community,bgrant0607/community,kubernetes/community