Document the nbresuse API

[jtpio]

Once the /metrics endpoint is back, we should document the API in the README.
We can do the same for the new /api/nbresuse/v1 endpoint that should be used in the long run.

The could also be a section to link to other repositories that depend on nbresuse and consume its endpoints:

• JupyterLab: https://github.com/jupyterlab/jupyterlab/blob/6f529d1b5701f83f20218e041b6c719028d2eea6/packages/statusbar/src/defaults/memoryUsage.tsx#L291
• https://github.com/jtpio/jupyterlab-system-monitor
  https://github.com/NERSC/jupyterlab-cpustatus