chore: copy existing README.md instead of creating a new one

[xianml]

What does this PR address?

Fixes #(issue)

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• [ ] Did your changes require updates to the documentation? Have you updated those accordingly? Here are documentation guidelines and tips on writting docs.
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[github-actions[bot]]

Package	Statements
bentoml	55.32% (9861 / 17824)
Summary	55.32% (9861 / 17824)