Document clear community membership guidelines

[anishasthana]

The current community member guidelines for Kubeflow are not clearly defined, which can definitely act as a barrier to entry for new contributors.

https://www.kubeflow.org/docs/about/contributing/ contains some high level details on the roles, but no clear criteria on what allows contributors to apply for community membership, reviewer status, or approver status.

The Argo Project seems to have pretty clearly defined membership criteria. We should do something similar for Kubeflow.

Kubernetes has another great one here: https://github.com/kubernetes/community/blob/master/community-membership.md#community-membership

cc @terrytangyuan

[terrytangyuan]

cc @kubeflow/kubeflow-steering-committee