docs(readme): make chaotic-kernel installation guide more explicit

[piyoki]

🐟 What?

According to the notes:

BREAKING CHANGE

To match the expected behavior of a flake, any non-derivation (e.g., linuxPackages_cachyos, scx_git, vulkanPackages_latest) now lives in legacyPackage instead of packages.
To help, some aliases were added: scx-full_git, linux_cachyos{,-hardened,-server} (kernel only without modules).
This won't affect users with our homeManagerModules.default, which will be able to use pkgs.linuxPackages_cachyos as usual.

See #949. Make installation guide more explicit.

Close #949

[PedroHLC]

Nope, pkgs is still the right place for people using our module, chaotic.legacyPackages is only for people using the package directly (without our modules and overlay)