Minor cleanup on contributing guide

[anishasthana]

Opening a WIP PR to provide a forum for us to discuss the for community membership guidelines topic.

This set of guidelines is heavily inspired by the Kubernetes guidelines at https://github.com/kubernetes/community/blob/master/community-membership.md?plain=1#community-membership

Closes #736

cc @kubeflow/kubeflow-steering-committee. I'll defer to your expertise/recommendations here!