Add astro react() integration gotcha to README

[JasperDeLanghe]

As suggested in #51

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[delucis]

Thanks for this and #51! I’ve actually been wondering if it makes most sense not to automatically include react() when adding the integration. This behaviour dates back to an early experiment that was much more tied to React components, but I suspect it could now work with or without @astrojs/react. In that case it would be better to leave that choice to each project instead of forcing a specific version of @astrojs/react on them.