Document YAML format

[lancelakey]

Currently in order to be able to use befog with an already existing and already running EC2 instance I would need to use befog to create a new throw-away EC2 instance to see an example befog YAML format in order to create a YAML file for my already running instance.

We need to document the befog YAML format so you don't have to create a throw-away instance with befog just to see the befog YAML format.