Mention this requires React<=17 in README

[AlbinoGeek]

See: https://github.com/Jungwoo-An/react-editor-js/issues/190#issuecomment-1105459267

This would prevent people using React 18 (the default) from opening duplicate issues.

Also, this would signal to @editorjs/editorjs that this doesn't support React 18.

Likewise, my example covers #58 as well (albeit, separate issue.)

[Jungwoo-An]

Thank you! I'll fix it asap