Docs about CORS: Accessing a cloud-local dask cluster as a JupyterHub user

I hoped that the #34 #31 should accomplish something allowing me to use this extension in a way that allowed me to write a scheduler URL that is only accessible from the jupyter server but not from the browser client.

For example, in a terminal in the jupyter server made available through a JupyterHub, I can `wget` to `http://dask-scheduler.dask:8787`, but from the browser, I cant access that URL. It is only available in the local network of the Jupyter server.

## Question

Was this supposed to work after #31 #34, or were those PRs not meant to accomplish this as I hoped they were with a lack for a proper understanding.