RFC: added performance tests for sparse getindex + a README file

In PR #7131 @ViralBShah suggested that I should add the `getindex` performance test to `perf/`.  Here a tidied up version of https://gist.github.com/mauro3/7f01931b5f25d7b397af.

Questions:
- I created a `README.md` as I tried to figure out how `perf/` works.  Maybe this could be useful?  It will need a read.
- more or fewer tests?
- which test should be included in `codespeed`?