Update README.md to add a deprecation notice

[kraenhansen]

As per the comment linked in the suggested change, this would make it more obvious that the package is no longer maintained. See #82 (comment) for context.

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[kraenhansen]

@mprobst I think it would it make sense to archive the repo and push a deprecation to NPM as well, to avoid additional users starting to rely on it.

[lumirlumir]

Hello. I've developed a new package to succeed clang-format, though the post was written some time ago. If you still need to use clang-format, please feel free to check it out.

https://github.com/lumirlumir/npm-clang-format-node