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 
Any reason we should link to One here rather than just making this repo the one source for supplemental guidelines as well? Since then they will be public and accessible by volunteers, and as far as I can see there's only one supplemental guideline on that page which could just be copied here.