docs: update Vietnamese input setup instructions in README

Add fcitx5-configtool addon and clarify that Unikey must be added via
fcitx5-configtool's Input Method tab, not through KDE Virtual Keyboard settings.

hosts/nixos/configuration.nix

@@ -4,9 +4,11 @@
   imports = [ ./hardware-configuration.nix ];
 
   # Bootloader
-  boot.loader.grub.enable = true;
-  boot.loader.grub.device = "/dev/sda";
-  boot.loader.grub.useOSProber = true;
+#  boot.loader.grub.enable = true;
+#  boot.loader.grub.device = "/dev/sda";
+#  boot.loader.grub.useOSProber = true;
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
 
   # Enable flakes and nix-command
   nix.settings.experimental-features = [ "nix-command" "flakes" ];
@@ -24,6 +26,7 @@
     fcitx5.addons = with pkgs; [
       qt6Packages.fcitx5-unikey
       fcitx5-gtk
+      fcitx5-configtool
     ];
   };