Add install ninja step

Author: tobiasdiez

CONTRIBUTING.md

@@ -5,13 +5,14 @@ Thank you for your interest in contributing to `meson-python`!
 ## How to Contribute
 
 1. **Fork the repository** and create your branch from `main`.
-2. **Setup development environment** using `pip install -e .[test]`.
+2. **Install Ninja** either using your system package manager (preferred) or via `pip install ninja`.
+3. **Setup development environment** using `pip install -e .[test]`.
    This will install as well all necessary dependencies for development and testing.
 
    It is recommended to use a virtual environment to avoid conflicts with system packages.
-3. **Make your changes** and add tests if applicable.
-4. **Run the test suite** to ensure all tests pass: `pytest` from the project root.
-5. **Submit a pull request** with a clear description of your changes.
+4. **Make your changes** and add tests if applicable.
+5. **Run the test suite** to ensure all tests pass: `pytest` from the project root.
+6. **Submit a pull request** with a clear description of your changes.
 
 ## Code Style
 - Follow the [PEP 8](https://www.python.org/dev/peps/pep-0008/) style guide, but with a line length of 127 characters.