Add READMEs for docker images

This PR adds READMEs to the `jupyter/singleuser` and `jupyter/systemuser` images explaining what they are and how they are meant to be used.

Also, I realized as I was writing these READMEs that `systemuserspawner.py` should probably just have `jupyter/systemuser` as the default image, because I don't think it will work with `jupyter/singleuser`, so I've included that change here too since it's so small.