Update documentation with a disclaimer that node already supports fetch

Hello,

First thank you so much for this library.

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

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

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

What do you think about this ?