add instructions to README for using text files with vscode-playground

[tombh]

Thank you. This just needs the commit message to be updated, see the CI report here. Basically something like: docs: using text files with vscode-playground

Edit: also needs a rebase of main to get the latest changes

[RossBencina]

@tombh done.

[dhruvmanila]

Not sure if this can be added in this PR itself but for inlay_hints.py server, we need the notebook document selector as well:

"pygls.client.documentSelector": [
    {
        "notebook": "*",
        "language": "python"
    }
]

[RossBencina]

This is just a fix for the README let's leave it at that.

[alcarney]

Thanks!