Write Helm chart for k8s deployment

This is more of a wishful desire for sake of completeness than an _actual_ requirement. It would be nice to have the option to deploy `acme-proxy` on Kubernetes clusters should users choose to do so. The project already provides a Docker image.

Deploying acme-proxy on Kubernetes comes with some gotcha's which should be documented

- Assuming ACME clients are located outside the k8s cluster, they can access acme-proxy via it's Ingress IP
- acme-proxy will perform challenge validation to client via it's Egress IP
- Depending on the architecture of the k8s cluster, acme-proxy may or may not be able to obtain a predictable egress IP which makes it challenging to configure the Firewall/HBF correctly specially in on-prem environments.

I would consider this issue to be a LOW priority and not a fundamental requirement for v1.0.0 release.