[k8sobserver] Add documentation for required RBAC permissions

[bacherfl]

Component(s)

extension/observer/k8sobserver

Describe the issue you're reporting

Currently the k8sobserver readme lacks a description of how to setup the required RBAC permissions (ClusterRole/ClusterRoleBinding) for the collector to be able to observe the K8s resources, such as in https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/k8sobjectsreceiver.
To make setting up this component easier, this should be added here as well.

I'm happy to work on a PR for this.

[github-actions]

Pinging code owners:

• extension/observer/k8sobserver: @rmfitzpatrick @dmitryax

See Adding Labels via Comments if you do not have permissions to add labels yourself.