refactored nixos directory & updated README

nixos/usb.nix

@@ -0,0 +1,25 @@
+{ pkgs, ... }:
+
+{
+  # USB Automounting
+  services.gvfs.enable = true;
+  # services.udisks2.enable = true;
+  # services.devmon.enable = true;
+
+  # Enable USB Guard
+  services.usbguard = {
+    enable = true;
+    dbus.enable = true;
+    implicitPolicyTarget = "block";
+    # FIXME: set yours pref USB devices (change {id} to your trusted USB device), use `lsusb` command (from usbutils package) to get list of all connected USB devices including integrated devices like camera, bluetooth, wifi, etc. with their IDs or just disable `usbguard`
+    rules = ''
+      allow id {id} # device 1
+      allow id {id} # device 2
+    '';
+  };
+
+  # Enable USB-specific packages
+  environment.systemPackages = with pkgs; [
+    usbutils
+  ];
+}