add flake

Author: deemp

README.md

@@ -37,3 +37,21 @@ format @"Hi {name}! Your favourite number is {number}" {name : "Bill"}
 
 The only requirement is that all the types in the record have `Show`
 instances.
+
+## Nix flake
+
+1. Install Nix.
+   - Use [single-user installation](https://nixos.org/download.html), then [enable flakes](https://nixos.wiki/wiki/Flakes).
+   - Alternatively, use an [unofficial](https://github.com/DeterminateSystems/nix-installer#the-determinate-nix-installer) installer.
+
+2. Run the default `devShell`.
+
+    ```console
+    nix develop
+    ```
+
+3. Test  the project
+
+    ```console
+    spago test
+    ```