Add documentation on running and adding benchmarks

[kdk]

We should have some documentation, ideally in test/benchmarks/README.md , that calls out:

• that we use ASV for benchmarking,
• how to write and contribute benchmarks ( Mostly referring to https://asv.readthedocs.io/en/stable/writing_benchmarks.html , but also calling out best practices like always setting seed_transpiler ),
• how to run benchmarks locally ( https://asv.readthedocs.io/en/stable/using.html#running-benchmarks plus necessary tweaks to asv.conf.json to point to a local repository ), and
• an example asv continuous command for e.g. comparing a PR branch to main.

[jakelishman]

This becomes relevant after #10546.