Benchmark documentation could use some improvements

[carols10cents]

Describe the bug

There's some confusing, inaccurate information in the benchmarks/README.md, and there isn't any documentation on adding a new benchmark.

To Reproduce

Try to follow the benchmarks readme and see that there's at least one command that doesn't do what the text says it should do.

Try to use the benchmarks readme to know how to add a new benchmark... which isn't possible because that section doesn't exist 😅

Expected behavior

Expected the documentation to be correct and contain the information I was looking for 😅

Additional context

Don't worry, PR incoming in a minute!