Revert "fix: remove nix support"

This reverts commit 499bb9b126.

README.md

@@ -47,6 +47,32 @@ On Alpine, `shadow` is needed for the user creation process during the install.
 # xbps-install -S libxml2-devel libcurl-devel
 ```
 
+### NixOS
+Add the flake to your inputs and import the module. That is all you need.
+Here's an example of using the modules in a flake:
+```nix
+# flake.nix
+{
+  inputs = {
+    omnisearch = {
+      url = "git+https://git.bwaaa.monster/omnisearch";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, omnisearch, ... }: {
+    nixosConfigurations.mySystem = nixpkgs.lib.nixosSystem {
+      modules = [
+        omnisearch.nixosModules.default
+        {
+          services.omnisearch.enable = true;
+        }
+      ];
+    };
+  };
+}
+```
+
 ### macOS (Homebrew)
 ```
 $ brew install libxml2 curl openssl pkg-config