Improve README

[andreufont]

The current README talks about fitter2 instead of talking about Vega, we should update this.

Also, it doesn't explain how to use Picca in Jupyter notebooks at NERSC (setup kernel, etc). I would suggest we include this, maybe under a "Running Picca at NERSC" sub-section.

[andreufont]

By the way, in the instructions for NERSC, it would be good to add a reference that conda environments should not be added to the HOME folder, and explain how to place these elsewhere