Update documentation with a disclaimer about node supporting already fetch

Hello,

First thank you so much for this library.

With node 21 supporting already fetch API (stable) and 18 ( experimental ) should we create a disclaimer in the repository about not needing to install node-fetch for those use cases?

This would save people from increasing their bundle size unnecessary if they are in a most recent node versions.

Maybe even a warning if installed in those runtimes could be an interesting feature.

What do you think about this ?