Instructions about installing in own namesapce

[vassilvk]

KubeMod must be installed in its own namespace.
The reason for this is that KubeMod's workload and configuration manifests must be excluded from KubeMod interceptions.

When installing with kubectl apply -f bundle.yaml this is trivial: https://github.com/kubemod/kubemod/blob/master/bundle.yaml#L5

When installing with Helm 3, we need to:

• Update the Helm installation instructions to ask the user to create a designated KubeMod namespace and install there. For example, see Longhorn installation instructions for Helm 3.
  Note that if the user is running Helm 3.2+, they can use --create-namespace alongside the --namespace option to create the target namespace if it doesn't exist.
• Figure out a way to apply admission.kubemod.io/ignore: "true" to the target namespace.