cmd/benchstat: add example benchmarking workflow to package comment

[nsajko]

README.md in cmd/benchstat was removed to prevent duplication and bit rot, because it was the duplicate of the package comment.

Fixes golang/go#23471

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[gopherbot]

This PR (HEAD: d9efb36) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/perf/+/188438 to see it.

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See the Wiki page for more info

[gopherbot]

This PR (HEAD: b8c3c35) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/perf/+/188438 to see it.

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See the Wiki page for more info

[gopherbot]

Message from Neven Sajko:

Patch Set 5:

Adding you as a reviewer because you said that would be OK in #23471

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