New wiki article about our extension mechanism

Hello, I wrote (its not finished yet) a wiki sheet to detail how our extensions mechanism work. This is not about how to write an extension :-p but it shows how the engine loads extensions, how it calls different hooks from them, and what are the differences between Zend extensions and PHP extensions. As I got plans to propose an RFC to rewrite this part of the engine (mainly to improve it, make it even more easy to manage from a PHP userland POV, and clean it from old dusty stuff), this first point should help anyone with little or no knowledge about that part. As a human, I make mistakes :) Feel free to edit the wiki page and add fixes. Julien.Pauli