Adding a PHP placeholder in the guidelines and MediaWiki stylize script

This adds a pointer to the MediaWiki guidelines as well as a submodule with [their stylize script](https://git.wikimedia.org/blob/mediawiki%2Ftools%2Fcode-utils.git/master/stylize.php) for PHP with instructions on how to use it.

I considered adding a [clean filter](http://git-scm.com/book/ch7-2.html) in git for doing this but that seems heavy handed. It someone wants to make sure that their code is formatting properly they can do so before staging it or as a separate cleanup commit.

/cc @Wikia/platform-team @lizlux 
That's fine with me. I copied it over in cb5f28c.