Readme: add short explanation of how tokei is so fast

[igorcafe]

Hi. I think it would be pretty cool if you add to the readme something explaining how tokei is so blazing fast.

I did a quick comparison with cloc and tokei was 20 times faster. I know cloc was made in perl, but the language itself is probably not the only reason for that.

Anyway, I just think people would like a high-level explanation of that.