README: add about Gentoo - #199

I'm not Gentoo user, so I don't know how things work there. Is Nulloy now included to the official repositories?

README is currently only meant for build instructions, but I will be happy to add Gentoo instructions on the main website.

Now Nulloy is not in the main Gentoo tree - but in a separate user tree (called Guru), after a few month (stabilization time) I will raise an issue to add Nulloy to the main tree.

Please add website to the Github, so it will be possible to create pull requests.

This is how user can connect Guru repo https://wiki.gentoo.org/wiki/Project:GURU

After that install as usual emerge --ask nulloy.

Please write instructions as a list of shell commands, so they could simply be pasted to terminal.

Done.

These instructions are no longer correct as the package has been removed from the GURU overlay in gentoo/guru@a68ce98 as part of Gentoo's migration to Qt6.

The related issue #230 can also be closed for the time being.

Qt6 rewrite is on the way, please bear with me.