docs: add CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Contributing to Filterable
+
+First off, thank you for considering contributing to Filterable. Every contribution — whether it's a bug report, a feature suggestion, or a pull request — is genuinely appreciated.
+
+---
+
+## Before You Start
+
+For anything beyond a small bug fix or typo, **please open an issue first** to discuss what you'd like to change. This avoids wasted effort if the direction doesn't align with the project's goals.
+
+---
+
+## Local Setup
+
+```bash
+git clone https://github.com/kettasoft/filterable.git
+cd filterable
+composer install
+```
+
+Run the test suite to make sure everything is working:
+
+```bash
+composer test
+```
+
+---
+
+## What You Can Contribute
+
+### Bug Fixes
+
+Open an issue describing the bug with a reproducible example, then submit a PR referencing that issue.
+
+### New Features
+
+Open an issue first and describe the feature and why it belongs in the package. Once discussed and approved, submit your PR.
+
+### New Engine
+
+Filterable is built around its engine architecture. If you want to add a new engine:
+
+1. Extend the `Engine` abstract class in `src/Engines/`
+2. Register it in the service provider
+3. Add full test coverage
+4. Add documentation for it
+
+Open an issue before starting so we can align on the design.
+
+### Documentation
+
+Improvements to the docs are always welcome. The documentation source lives in the `docs/` branch.
+
+---
+
+## Pull Request Guidelines
+
+- **One PR per feature or fix** — keep changes focused
+- **Write tests** for any new behavior
+- **Follow existing code style** — PSR-12
+- **Reference the related issue** in your PR description
+- **Update documentation** if your change affects public-facing behavior
+
+---
+
+## Commit Message Format
+
+Follow this format:
+
+```
+type: short description
+```
+
+Types:
+
+| Type       | When to use                         |
+| ---------- | ----------------------------------- |
+| `feat`     | New feature                         |
+| `fix`      | Bug fix                             |
+| `docs`     | Documentation only                  |
+| `refactor` | Code change with no behavior change |
+| `test`     | Adding or updating tests            |
+| `chore`    | Build process, dependencies, config |
+
+Examples:
+
+```
+feat: add support for custom operators in Ruleset engine
+fix: resolve depth limit not being enforced in Tree engine
+docs: add Expression engine examples to README
+```
+
+---
+
+## Running Tests
+
+```bash
+# Run all tests
+composer test
+
+# Run a specific test file
+./vendor/bin/phpunit tests/Engines/RulesetEngineTest.php
+```
+
+Please make sure all existing tests pass before submitting a PR, and add tests for any new behavior you introduce.
+
+---
+
+## Reporting a Bug
+
+When opening a bug report, please include:
+
+- PHP and Laravel version
+- Package version
+- A minimal reproducible example
+- What you expected vs what actually happened
+
+---
+
+## Code of Conduct
+
+Be respectful. Constructive feedback is welcome, personal attacks are not. This project follows the [Contributor Covenant](https://www.contributor-covenant.org/).
+
+---
+
+## Questions?
+
+Open a [GitHub Discussion](https://github.com/kettasoft/filterable/discussions) or an issue and we'll get back to you.