Add preferred documentation language

CONTRIBUTING.md

 
 Also, please ensure your pull request adheres to the following guidelines:
 
-* Software that is PHP 5.5+, Composer-installable, PSR compliant, united tested and well documented is preferred.
+* Software that is PHP 5.5+, Composer-installable, PSR compliant, united tested and well documented in English is preferred.
 * Please search previous suggestions before making a new one, as yours may be a duplicate.
 * Enter a meaningful pull request description.
 * Please make an individual commit for each suggestion in a pull request.