Advertise HHVM 3.5+ [skip ci]

Author: thekid

README.md

@@ -3,6 +3,7 @@ XP Framework Core
 [![Build Status on TravisCI](https://secure.travis-ci.org/xp-framework/core.png)](http://travis-ci.org/xp-framework/core)
 [![BSD Licence](https://raw.githubusercontent.com/xp-framework/web/master/static/licence-bsd.png)](https://github.com/xp-framework/core/blob/master/LICENCE.md)
 [![Required PHP 5.4+](https://raw.githubusercontent.com/xp-framework/web/master/static/php-5_4plus.png)](http://php.net/)
+[![Required HHVM 3.5+](https://raw.githubusercontent.com/xp-framework/web/master/static/hhvm-3_5plus.png)](http://hhvm.com/)
 [![Latest Stable Version](https://poser.pugx.org/xp-framework/core/version.png)](https://packagist.org/packages/xp-framework/core)
 
 This is the XP Framework's development checkout