[wiki] Add home-manager's way to enable snixembed

[DamienCassou]

The wiki explains how to install snixembed on several distributions. It lacks information about Home Manager which recently got support for snixembed.

It seems that the wiki is read-only so I can't make the change myself. Here is some text you could use if you want:

Nix with Home Manager

You just have to add the following to your Home Manager configuration file:

{
  ...
  safeeyes.enable = true;
  snixembed = {
    enable = true;

    beforeUnits = [
      # https://github.com/slgobinath/SafeEyes/wiki/How-to-install-backend-for-Safe-Eyes-tray-icon
      "safeeyes.service"
    ];
  };
}

Now reboot, and Safe Eyes should work!