Comparison with `unshare` from util-linux

[septatrix]

The util-linux packages are an official set of programs developed under the Linux organization. They are also preinstalled on pretty much every system as they contain e.g. agetty, login, and mount. They also ship a binary called unshare which is very similar to bubblewrap. It would be great to add a section for it under the "Related project comparison" part of the README