add CONTRIBUTING.md

Author: darsnack

CONTRIBUTING.md

@@ -0,0 +1,42 @@
+# Contributing to Cheese3D
+
+Thanks for your interest in contributing!
+
+## Getting started
+
+1. Fork and clone the repository.
+2. Install [Pixi](https://pixi.sh), then set up the environment:
+   ```bash
+   pixi install
+   ```
+3. Create a branch for your changes:
+   ```bash
+   git checkout -b my-change
+   ```
+
+## Making changes
+
+- If you change behavior or add a feature, update the docs under `docs/source/`.
+- Write a clear commit message describing the *why* of your change.
+
+## Building the docs
+
+```bash
+pixi run docs        # build once
+pixi run docs-serve  # build and serve with live reload
+```
+
+## Submitting a pull request
+
+1. Push your branch to your fork.
+2. Open a pull request against `main`.
+3. Describe what changed and why, and link any related issues.
+
+## Reporting bugs
+
+Open an issue on GitHub and include:
+
+- A short description of the problem
+- Steps to reproduce
+- Expected vs. actual behavior
+- Your OS, Python version, and Cheese3D version