Add an example about status bar items

[jtpio]

Another way to extend the JupyterLab interface is to add a new item or indicator to the status bar:

There could be an example in this repo to explain how to do this, something similar to this plugin in core lab: https://github.com/jupyterlab/jupyterlab/blob/f0cb758333df2bb622689bcc8b92e63bb9c533c6/packages/notebook-extension/src/index.ts#L314-L347

With a link to the JupyterLab documentation: https://jupyterlab.readthedocs.io/en/stable/developer/extension_points.html#status-bar