doc: contributing with shell.nix

Author: Chemaclass

.github/CONTRIBUTING.md

@@ -94,6 +94,20 @@ docker run --rm -it -v "$(pwd)":/project -w /project alpine:latest \
     sh -c  "apk add bash make shellcheck git && make test"
 ```
 
+##### NixOS
+
+If you are using NixOS, you can enter a shell with all the required tools using the provided `shell.nix`:
+
+```bash
+nix-shell --pure --run "./bashunit --simple"
+```
+
+Alternatively, if you just need Bash for a one-time execution and don’t want to rely on the `shell.nix` file:
+
+```bash
+nix-shell -p bash --run "./bashunit --simple"
+```
+
 ## Coding Guidelines
 
 ### ShellCheck

shell.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.bashInteractive
+    pkgs.git
+    pkgs.curl
+    pkgs.perl
+  ];
+}