Initialize CONTRIBUTING guide

CONTRIBUTING.md

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+## Requirements
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+- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](https://pear.php.net/package/PHP_CodeSniffer).
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+- **Add tests!** - Your patch won't be accepted if it doesn't have tests.
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+- **Consider our release cycle** - We try to follow [SemVer v2.0.0](https://semver.org/). Randomly breaking public APIs is not an option.
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+- **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.
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+- **Send coherent history** - Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash them](https://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
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+**Happy coding**!