Add CONTRIBUTING.md (#403)

Author: jayvdb

CONTRIBUTING.md

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+# Contributing to Butane
+
+Thank you for your interest in contributing to Butane!
+This guide will help you set up your development environment and get started.
+
+## Development Environment Setup
+
+### Using mise (Recommended)
+
+This project uses [mise](https://mise.jdx.dev/) to manage development tools needed for development.
+
+#### Installing mise
+
+**Package Managers (Recommended):**
+
+mise is available in many package managers. Choose the one for your system:
+
+- **Nix/NixOS:** `nix-env -iA nixpkgs.mise`
+- **macOS (Homebrew):** `brew install mise`
+- **macOS (MacPorts):** `sudo port install mise`
+- **Windows (Chocolatey):** `choco install mise-en-place`
+- **Windows (Scoop):** `scoop install mise`
+- **Arch Linux:** `pacman -S mise`
+- **Alpine Linux:** `apk add mise`
+- **FreeBSD:** `pkg install mise`
+
+For a complete list of package managers and distributions,
+see [mise on Repology](https://repology.org/project/mise/versions).
+
+##### Alternative: Install via Cargo
+
+If you already have Rust installed, you can install mise using cargo:
+
+```bash
+cargo install mise
+```
+
+**Note:** When installing via cargo, you'll need to manually set up its shell integration.
+
+##### Alternative: Quick Install Script
+
+> ⚠️ **Security Warning:** This alternative is not secure. We recommend using a package manager instead.
+
+If you prefer the installation script:
+
+**macOS/Linux:**
+
+```bash
+curl https://mise.run | sh
+```
+
+**Windows (PowerShell):**
+
+```powershell
+irm https://mise.run | iex
+```
+
+**For other installation methods**, see the [official mise documentation](https://mise.jdx.dev/getting-started.html).
+
+#### Installing Project Tools
+
+Once mise is installed and shell integration is activated, navigate to the project directory and run:
+
+```bash
+mise install
+```
+
+This will automatically install all required tools defined in `.mise.toml`.
+
+### Without mise
+
+If you prefer not to use mise, you'll need to manually install:
+
+1. **Rust** (stable) - via [rustup](https://rustup.rs/)
+2. **PostgreSQL** - for running tests
+3. **SQLite** - for running tests
+4. **Make** - for running development commands
+5. Other development tools as needed - they will typically be added to `.mise.toml`.
+
+## Building the Project
+
+```bash
+cargo build
+```
+
+## Running Tests
+
+Run all tests:
+
+```bash
+cargo test
+```
+
+Run tests for a specific package:
+
+```bash
+cargo test -p butane
+cargo test -p butane_core
+```
+
+## Code Quality
+
+Before submitting a PR, you may use the following commands locally to ensure that your code passes all checks:
+
+```bash
+make check
+```
+
+Or run individual checks:
+
+```bash
+# Format code
+make fmt
+
+# Run linter
+make lint
+
+# Check for typos
+make spellcheck
+
+# Check formatting and editor config
+make check-fmt
+
+# Check documentation
+make doclint
+```
+
+## Project Structure
+
+- `butane/` - Main library crate
+- `butane_core/` - Core functionality
+- `butane_cli/` - Command-line interface
+- `butane_codegen/` - Code generation macros
+- `butane_test_helper/` - Test utilities
+- `examples/` - Example projects
+- `docs/` - Documentation
+
+## Database Migrations
+
+This repository contains generated migrations for the examples. To regenerate migrations:
+
+```bash
+make regenerate-example-migrations
+```
+
+## Submitting Changes
+
+1. Fork the repository
+2. Create a feature branch (`git checkout -b feature/my-feature`)
+3. Make your changes
+4. Run tests and quality checks
+5. Commit your changes with clear commit messages
+6. Push to your fork
+7. Open a Pull Request
+
+## Getting Help
+
+- Open an issue on [GitHub](https://github.com/Electron100/butane/issues)
+- Check existing issues and PRs
+- Review the [documentation](https://docs.rs/butane)
+
+Thank you for contributing to Butane!