docs: add conventional commit specification to CONTRIBUTING

Author: Goodmain

CONTRIBUTING.md

@@ -14,5 +14,6 @@ Please review these guidelines before submitting any pull requests.
 
 * Please follow the [PSR-12 Coding Style Guide](https://www.php-fig.org/psr/psr-12/).
 * Send a coherent commit history, making sure each individual commit in your pull request is meaningful.
+* Please follow the [Conventional Commits specification](https://www.conventionalcommits.org/en/v1.0.0/).
 * 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/).