Wordsmith note in README about limited support on the web

[brettcannon]

https://github.com/microsoft/vscode-python/blob/main/README.md has a NOTE about the web support, but I did it very quickly and so it could probably stand to be improved.

[karrtikr]

We can possibly link to the wiki here for more details: https://github.com/microsoft/vscode-python/wiki/Partial-mode

This wiki is also opened when Learn more is clicked here (#17658):