Added gentoo installation instructions

[medanisjbara]

Gentoo installation instructions

Type of change

• [ ] Bug fix
• [ ] Feature
• [X] Documentation update

Description

I copied the instructions on nheko and replaced the name of the package.

Note that due to the forth regulation of the guru regulations. People on stable will have to accept the unstable keyword for the package (which is not in the installation instructions)

[justchokingaround]

need for someone to test this on gentoo pls

[medanisjbara]

@justchokingaround I'm actually the maintainer of the guru package on gentoo 😅. Though there's nothing wrong with more testing ig.