Add Homebrew installation instructions in README.md
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I just [added bench to Homebrew](https://github.com/Homebrew/homebrew-core/pull/19215). I would be nice to mention that on README.
I also removed the mention of pre-built binaries since it seems that releases are no longer tagged or no longer pushed to GitHub, let alone binaries.
bench 1.0.6 (new formula)
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- [x] Have you followed the [guidelines for contributing](https://github.com/Homebrew/homebrew-core/blob/master/CONTRIBUTING.md)?
- [x] Have you checked that there aren't other open [pull requests](https://github.com/Homebrew/homebrew-core/pulls) for the same formula update/change?
- [x] Have you built your formula locally with `brew install --build-from-source <formula>`, where `<formula>` is the name of the formula you're submitting?
- [x] Does your build pass `brew audit --strict <formula>` (after doing `brew install <formula>`)?
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