busybox descriptions

@Moghedrin @tianon @fredlf, can I get a read through?

`content.md` is the main file, but also looking for suggestions on `README-short.txt`.
I'm going to nitpick, but I wouldn't call busybox a "distribution". I'd rather say (instead of "is one of..."):

is a very good ingredient to craft space-efficient distributions.

is a very