docs(readme): improve Nix development environment setup instructions

- Add Nix installation command using experimental installer
- Add instructions for enabling flakes if not already enabled
- Clarify direnv activation (use existing .envrc)
- Link to flake.nix for reference
- Reorganise useful commands into separate section

Author: ryoppippi

README.md

@@ -337,20 +337,27 @@ For more examples, check out the [examples/](examples/) directory:
 
 ### Using Nix (Recommended)
 
-[Nix](https://nixos.org/) with flakes provides a consistent development environment:
+This project includes a Nix flake for reproducible development environments. All development tools are defined in [flake.nix](./flake.nix) and provided via Nix.
+
+#### Installing Nix
 
 ```bash
-# Enter development environment (auto-installs dependencies and git hooks)
-nix develop
+# Install Nix with flakes enabled (if not already installed)
+curl --proto '=https' --tlsv1.2 -sSf -L https://artifacts.nixos.org/experimental-installer | \
+  sh -s -- install
 
-# Or with direnv (recommended for automatic activation)
-direnv allow
+# If flakes are not enabled, enable them with:
+mkdir -p ~/.config/nix && echo "experimental-features = nix-command flakes" >> ~/.config/nix/nix.conf
+```
 
-# Format code
-nix fmt
+#### Activating the Development Environment
 
-# Run checks
-nix flake check
+```bash
+# Automatic activation with direnv (recommended)
+direnv allow
+
+# Or manual activation
+nix develop
 ```
 
 The Nix development environment includes:
@@ -360,6 +367,16 @@ The Nix development environment includes:
 - Git hooks (treefmt + ty) auto-configured
 - Consistent environment across all platforms
 
+#### Useful Commands
+
+```bash
+# Format code
+nix fmt
+
+# Run checks
+nix flake check
+```
+
 ## License
 
 Apache 2.0 License