Adding readme on how to run benchmarks

tests/benchmarks/README.md

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+This folder contains benchmarks written using `pytest` and profiled using `pyinstrument`.
+
+# Running the benchmarks
+
+Open this repository in VSCode and wait for the devcontainer to start up.
+
+Then you can run all the benchmarks with this command:
+
+`python -m pytest tests/benchmarks`
+
+Or use the keyword argument to run just one benchmark test:
+
+`python -m pytest tests/benchmarks -k test_process_indexed_function`
+
+When you run that test, a profile will also be saved in the `tests/benchmarks/.profiles` folder, in a file named after the profiled function. Open the file in a browser to see the profile.
+
+If you run a benchmark test multiple times (either on same code or different versions of the code), you probably want to save it.
+
+Either pass in `--benchmark-autosave` to save to an auto-generated filename or pass in `--benchmark-save=YOURNAME` to save with your specified name in the filename. All benchmark files will always start with a counter, beginning with 0001.
+
+Once saved, compare using the `pytest-benchmark` command and the counter numbers:
+
+`pytest-benchmark compare 0004 0005`
+
+You can sort the comparison using `--sort`, save it to a CSV using `--csv`, or save it to a histogram with `--histogram`.
+More details available in the [pytest-benchmark reference](https://pytest-benchmark.readthedocs.io/en/latest/usage.html#comparison-cli).