Add inline notes to jupyterhub_config.py to reflect its purpose

[consideRatio]

I saw this repo and wasn't sure how things worked, and remain unsure. Is jupyterhub_config.py just used for local development, or is it also used by 2i2c's infrastructure cloning this repo somehow?

This is a question relating to 2i2c-org/infrastructure#1501 as finding the jupyterhub_config.py made me think maybe this is a way for hub champions to configure various template vars themselves or similar, at the same time, I think maybe it isn't as I recall seeing this kind of configuration in 2i2c-org/infrastructure for the specific hubs.

My understanding now is that jupyterhub_config.py is just for local testing purposes, but its unclear and it would be valuable to have that clearly noted in this repo. Both in README.md and the jupyterhub_config.py file itself.