Update package README.md

[Rall3n]

Seems like the packages README.md file was forgotten during the upgrade to v5 (#4559 (comment)).

This PR just copies the contents of the ./README.md to the ./packages/react-select/README.md file.

In the future, we should either keep the packages README.md up to date or as the only file, or look at a way to synchronize when building.

[changeset-bot]

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Latest commit: 8c75d85

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