Added zend-diactoros

README.md

 * [PHP VCR](http://php-vcr.github.io/) - A library for recording and replaying HTTP requests.
 * [Requests](https://github.com/rmccue/Requests) - A simple HTTP library.
 * [Retrofit](https://github.com/tebru/retrofit-php) - A library to ease creation of REST API clients.
+* [zend-diactoros](https://github.com/zendframework/zend-diactoros) - PSR-7 HTTP Message implementation.
 
 ## Scraping
 *Libraries for scraping websites.*