docs(readme): add helm chart reference for k8s to notes section

[pascaliske]

Hi @crazy-max,

Thank you for building and maintaining this container for the cloudflared tool! 🚀

I don't know if you find this useful but I built a helm chart for this container for anyone who wants to install it inside an Kubernetes cluster. It would be an honor for me to be mentioned on your project!

Feel free to close this PR if you don't want to mention it! 🙂