Document how to test JupyterHub images locally

[jnywong]

@yuvipanda has developed a way to test a JupyterHub image on a local machine: https://github.com/yuvipanda/test-jupyterhub-image-locally

This would provide value for Hub Administrators by documenting this on the Hub Service Guide. There have been a couple of cases where communities have demonstrated a need for this guide.