Add a table of benchmark results to the ideas repo README

[ericsnowcurrently]

We've been uploading the JSON files for various benchmark runs to the benchmark-results directory. However, this isn't so useful without comparisons. So we'll add a table showing the geometric mean of each of our results, compared against 3.10.4.

(Thanks to @gvanrossum for the suggestion.)

[ericsnowcurrently]

Inspired by python/devguide#884, I'm going to look into uploading a CSV file (in benchmark-results/) and then embed it in the README.

[ericsnowcurrently]

This has been done in #69 and #70.