refactored nixos directory & updated README

nixos/gc.nix

@@ -0,0 +1,16 @@
+{ ... }:
+
+{
+  # Optimize storage and automatic scheduled GC running
+  # If you want to run GC manually, use commands:
+  # `nix-store --optimize` for finding and eliminating redundant copies of identical store paths
+  # `nix-store --gc` for optimizing the nix store and removing unreferenced and obsolete store paths
+  # `nix-collect-garbage -d` for deleting old generations of user profiles
+  nix.settings.auto-optimise-store = true;
+  nix.optimise.automatic = true;
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 7d";
+  };
+}