contributing documentation

Author: faissaloux

Diff for: CONTRIBUTING.md

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+# CONTRIBUTING
+
+Contributions are welcome, and are accepted via pull requests.
+Please review these guidelines before submitting any pull requests.
+
+## Process
+
+1. Fork the project
+1. Create a new branch
+1. Code, test, commit and push
+1. Open a pull request detailing your changes.
+
+## Guidelines
+
+* Please ensure the coding style running `composer lint`.
+* Send a coherent commit history, making sure each individual commit in your pull request is meaningful.
+* You may need to [rebase](https://git-scm.com/book/en/v2/Git-Branching-Rebasing) to avoid merge conflicts.
+* Please remember that we follow [SemVer](http://semver.org/).
+
+## Setup
+
+Clone your fork, then install the dev dependencies:
+```bash
+composer install
+```
+## Lint
+
+Lint your code:
+```bash
+composer lint
+```
+## Tests
+
+Run all tests:
+```bash
+composer test
+```
+
+Check types:
+```bash
+composer test:types
+```
+
+Unit tests:
+```bash
+composer test:unit
+```