Add documentation

Actual documentation. The stuff that the code can't really document itself. Intention is to make this helpful for new starters and also for those who weren't at LP when a lot of the decisions were made and so don't have the back story.

This is just a starting point, the stuff that I thought would be generally helpful. It's over to you to actually make it better.

Probably easier to read this using github (or in rizzo ;) than in this diff: https://github.com/lonelyplanet/rizzo/tree/64f1f2db5f565df04e67273a00375a2e9620c619/app/docs