Add Zephir

README.md

 ## Extensions
 *Libraries to help build PHP extensions.*
 
+* [Zephir](https://github.com/phalcon/zephir) - A compiled language between PHP and C++ for developing PHP extensions.
 * [PHP CPP](http://www.php-cpp.com/) - A C++ library for developing PHP extensions.
 
 ## Miscellaneous