chore(readme): Add Gentoo installation instructions

Author: gyakovlev

README.md

@@ -78,6 +78,7 @@ If you want to uninstall the extension, you may invoke `make uninstall`, and the
 ### Packaging status
 
 - [Fedora](https://src.fedoraproject.org/rpms/gnome-shell-extension-pop-shell/): `sudo dnf install gnome-shell-extension-pop-shell` 
+- [Gentoo](https://packages.gentoo.org/packages/gnome-extra/gnome-shell-extension-pop-shell): `emerge gnome-shell-extension-pop-shell`
 - [Arch Linux](https://aur.archlinux.org/packages/?O=0&K=gnome-shell-extension-pop-shell) (Using Yay as AUR helper): 
     - `yay -S gnome-shell-extension-pop-shell`
     - For precompiled binary version: `yay -S gnome-shell-extension-pop-shell-bin`