Update README for POSIX config?

[therealnlee]

I'm using Jupyterlab on jupyterhub now, and tried to show CPU usage (which I believe the default value is False) and the config below (as in current README) did not work properly for jupyterlab

# POSIX shell
jupyter lab --NotebookApp.ResourceUseDisplay.mem_limit=$(( size_in_GB *1024*1024*1024)) \
            --NotebookApp.ResourceUseDisplay.track_cpu_percent=True \
            --NotebookApp.ResourceUseDisplay.cpu_limit=$(( number_of_cpus ))

After I changed NotebookApp to ServerApp for these configs, I got what I wanted. I would be much better to update README for the users like me. Thanks!