Add "Understanding Commotion" to CCK or Guides section

[andygunn]

Commotion has needed an overview guide for a while - something like "Understanding Commotion" or "How Commotion Works". It was outlined quite a while ago, but no one took it on:
https://docs.google.com/a/opentechinstitute.org/document/d/1M8hzFNKoHbNZ8sW-C85RKEvevTgb0xmoDPbU81Rf7FM/edit#

@jheretic @dismantl - I think it is time someone wrote this out, and created diagrams. To get this right, I think a dev or someone with a deeper understanding of the innards should write it up.