Explanation of directory structure

[jcrben]

I came to this repo because I was looking to fix a broken link in the website.

As it turns out, most of the content is not in this repo but rather in https://github.com/phpDocumentor/phpDocumentor2/tree/develop/docs which is pulled into here by https://github.com/phpDocumentor/phpdoc.org/blob/master/bin/get-docs.sh

That's not very intuitive, and should be explained upfront in the README.