readme update obsolete option

[acquaregia]

Hi
the line in
./sample_from_model.py -m gdb13_exp/models/model.trained.100 --with-likelihood
should be changed into
./sample_from_model.py -m gdb13_exp/models/model.trained.100 --with-nll

or change it in the code to reflect the readme

and add the meaning of the acronym somewhere in the file
negative log-likelihood (NLL)

Keep on with the great work!
marco