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