Benchmark envelop-only vs deep field matching

## What is your idea?

Discussed with @baldawar that it would be nice to know, e.g. in `README.md` whether it's preferable to pattern match on envelop (first-level) fields only or whether there's no significant impact on a match at a lower-level, e.g. `Detail.someField`.

## Would you be willing to make the change?

Maybe, need advise on Java benchmarking.

## Additional context

n/a

Start by reviewing README.md and locating the project's Java matching and benchmarking entry points. Compare envelope-only matching with matching on a nested field such as Detail.someField, then document the measured impact and recommendation in README.md.

Written by the indexing model from the issue text.