Add Zxcvbn-php

Add @bjeavons' Zxcvbn-php library

README.md

 * [IniScan](https://github.com/psecio/iniscan) - A tool that scans PHP INI files for security.
 * [SensioLabs Security Check](https://security.sensiolabs.org/) - A web tool to check your Composer dependencies for security advisories.
 * [Zed](https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project) - An integrated penetration testing tool for web applications.
+* [Zxcvbn PHP](https://github.com/bjeavons/zxcvbn-php) - A realistic PHP password strength estimate library based on Zxcvbn JS.
 
 ## Code Analysis
 *Libraries and tools for analysing, parsing and manipulation codebases.*