[dev-overlay] docs: add readme (#76396)
This PR added the README for the dev overlay for future contributors.
[**View
Preview**](https://github.com/vercel/next.js/blob/9136b280a3e194b81c1ea9e4aa7236eeb895a843/packages/next/src/client/components/react-dev-overlay/README.md)
Closes NDX-656
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Co-authored-by: Lee Robinson <lee@leerob.com>
