README: Mention Guix package-manager installation option

Hello,

Thank you for creating and maintaining just, it’s a useful tool!

I wanted to suggest a small addition to the README: in GNU Guix, just is already packaged and can be installed with:

`guix install just
`

Reference: https://packages.guix.gnu.org/packages/just/

This might be useful to add under the installation instructions, alongside other package managers.

Thanks!