docs: nixos install example

docs/abra/install.md

@@ -26,6 +26,31 @@ wget -q -O - https://install.abra.coopcloud.tech | bash
 curl https://install.abra.coopcloud.tech | bash
 ```
 
+### NixOS
+
+The install example is based on the [using nix flakes wiki page](https://nixos.wiki/wiki/flakes#Using_nix_flakes_with_NixOS).
+1. Add the flake to your inputs
+```nix
+inputs = {
+  abra = {
+    url = "git+https://git.coopcloud.tech/toolshed/abra.git";
+  };
+};
+```
+2. Add abra package to system packages, replace _x86_64-linux_ with the value according to your system 
+```nix
+{
+  pkgs,
+  abra,
+  ...
+}:
+{
+  environment.systemPackages = with pkgs; [
+    abra.packages.x86_64-linux.default
+  ];
+}
+```
+
 ## Release candidate
 
 ### Wget