Add a chart comparing Argh to existing parsers

If Argh is "optimized for code size", then it only makes sense to show a comparison with other existing parsers, similar to the chart in the README for [pico-args](https://lib.rs/crates/pico-args).

(Though, ideally, also with a quick run-down of how the features and APIs compare.)

The pico-args chart covers bare Clap, Clap+structopt, pico-args, and gumdrop, though [argparse](https://lib.rs/crates/argparse) also shows up reasonably high in search results.