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README.md

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+This is my NixOS configuration. I use it to manage:
+* some vps serving some websites and services (misskey, synapse), etc.
+* my laptop (Lenovo R9000P 2023), and my tablet (One Netbook One Mix 4).
+* some cluster for scientific computing (vasp, lammps, etc).
+With the following highlights:
+* All binary is compiled for specific CPU (`-march=xxx`, like that on Gentoo). 
+* All packages and configurations are managed by Nix, as much reproducible as possible.
+
+## Using overlay
+
+An overlay is provided through `outputs.overlays.default`, you could use it in your `configuration.nix` like this:
+
+```nix
+{
+  inputs.chn-nixos.url = "github:CHN-beta/nixos";
+  outputs.nixosConfigurations.my-host = inputs.nixpkgs.lib.nixosSystem
+  {
+    modules = [({pkgs, ...}: { config =
+    {
+      nixpkgs.overlays = [ inputs.chn-nixos.overlays.default ];
+      environment.systemPackages = [ pkgs.localPackages.vasp.intel ];
+    };})];
+  };
+}
+```
+