Add some documentation on what are the minimum required env vars needed to run

While doing the docker PR I took some time messing around to check what where really the minimum env vars that I needed to run the project locally.
It would be nice if we had something like a table mapping what one should setup to run at local env and another on how to run on heroku.

I think this is pretty much covered now, as no env vars are needed and the documented make commands just work.