Add skipper-php

README.md

 * [PHPUnit](https://github.com/sebastianbergmann/phpunit) - A unit testing framework.
 * [PHPUnit Polyfills](https://github.com/Yoast/PHPUnit-Polyfills/) - Simplifies running PHPUnit tests on multiple PHPUnit versions.
 * [Prophecy](https://github.com/phpspec/prophecy) - A highly opinionated mocking framework.
+* [skipper-php](https://github.com/get-skipper/skipper-php) - Real-time test execution control via Google Spreadsheet, enabling instant toggle without code changes.
 * [VFS Stream](https://github.com/bovigo/vfsStream) - A virtual filesystem stream wrapper for testing.
 
 ### Continuous Integration