Update README

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit ce569de):

https://flutterkakgi-2023-conf-app-dev--pr92-feat-update-readm-us6v31t9.web.app

_{(expires Tue, 05 Sep 2023 14:34:41 GMT)}

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[koji-1009]

@FlutterKaigi/conference-app-2023 レビューをお願いします！