Add `baseUrl` notices to readme

[hyperupcall]

This updates the documentation to add a notice that baseUrl may need to be set manually, for places where it is relevant.

But, checking the docs for baseUrl, it seems that it is only recommended for use with AMD module loaders (since TypeScript 4.1, it is no longer required when using 'paths`). So, I'm thinking it might be better just to remove the properties all together (with no documentation update)? That will still address the "confusion" from the linked issue, and not add "notices" that do not apply for most people.

Closes #223

[orta]

I've always tried to recommend C&Ping directly from the source, and so it's likely that if we removed the baseUrl, someone would bring it back - which means either post-processing the bases for publishing or leaving a note like this

So, I think this is a pretty reasonable solution overall and I'd be fine with it as the answer

[orta]

Its kind of the same tension in https://github.com/tsconfig/bases/issues/207

[hyperupcall]

@orta I see, that makes sense - README note it is then