add docs for k8s kubeconfig, defaultNamespace

[mukundansundar]

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Description

Based on PR dapr/components-contrib#3060

Issue reference

Closes #3669

[mukundansundar]

@ItalyPaleAle Please review PR

[msfussell]

Any consequences if this value is not set?

[mukundansundar]

No, defaultNamespace is not a required parameter.

[mukundansundar]

namespace is a required parameter to get secrets ....
either through request metadata, or though NAMESPACE env var.
https://github.com/dapr/components-contrib/blob/master/secretstores/kubernetes/kubernetes.go#L112-L126

func (k *kubernetesSecretStore) getNamespaceFromMetadata(metadata map[string]string) (string, error) {
	if val, ok := metadata["namespace"]; ok && val != "" {
		return val, nil
	}

	if val := os.Getenv("NAMESPACE"); val != "" {
		return val, nil
	}

	if k.md.DefaultNamespace != "" {
		return k.md.DefaultNamespace, nil
	}

	return "", errors.New("namespace is missing on metadata and NAMESPACE env variable, and no default namespace is set")
}

[msfussell]

LGTM