Add namespace warnings to README and rbac.yaml

[sryabkov]

what

Add namespace warnings toREADME.rst and deploy/common/rbac.yaml

why

• The current deployment manifests assume that kube-janitor is deployed to the default Kubernetes namespace. If kube-janitor is deployed to a different namespace, this can lead to problems described in 403 Client Error: Forbidden for url #35, at least on GKE.
• The author solicited this PR over here

notes

• This is a documentation/comment only PR. It contains no code or configuration changes.

[hjacobs]

Thanks!