Add installation instructions for Gentoo Linux

[Flowdalic]

No description provided.

[lefou]

Yeah, why not.

Besides, I think any Gentoo user should have figured it out by herself, as it's regular procedure:

$ eix mill
* dev-java/mill-bin
     Verfügbare Versionen:   ~0.9.9
     Startseite:             https://com-lihaoyi.github.io/mill/
     Beschreibung:           A Java/Scala build tool
...
$ emerge mill-bin
....

Also, I think this one installs a fixed version (which is perfectly in the spirit of Gentoo, which I'm using too, by the way), whereas most developers prefer to have a dynamic version, which means: Use (and ad-hoc download) whatever mill version is specified in .mill-version. This will likely require to use a script like this or lefou/millw.

[lefou]

Thanks!