[enhancement] Include in the documentation an example to run the tool on kubernetes

Running the diagnostic tool (`--type api`) when elasticsearch is running on Kubernetes is quite straight forward following this trick:

```
# Use kubectl port-forward to connect directly to the remote elasticsearch service (or pod)
kubectl port-forward service/elastic-k8s-es-http 9200:9200

# Then you can run the tool towards localhost (or 127.0.0.1) with --type api
./diagnostics.sh --type api -u your_username -p -s --noVerify -h localhost
```

Of course the ports used in the previous example could be adapted.

As a lot of users end up needing this now that Kubernetes is widely used, it might be interesting to add this way in a section of the doc.
Of course if a `local` execution is needed then this workaround is not valid and the tool should be either included in the docker image, or run directly from the kubernetes worker (normal users won't do that in kubernetes clusters), or downloaded directly from the running container / pod. But in general I have never felt this need.