Add documentation regarding development

[rgaiacs]

When developing Zero to JupyterHub with Kubernetes, one of the problems that I have is that the tag for the container image is set-by-chartpress, see https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/d55c8e35033644ea8caadb9bf0258492cde689c1/jupyterhub/values.yaml#L90. This means that when I run helm the deployment o the kubernetes cluster tries to get a container image with the tag set-by-chartpress that does not exists.

Can the README be expanded to include a step by step section for novices on how configure their environment for contributing to projects using chartpress?

[consideRatio]

Sounds fine to me!