Add orienting material: "wtf is a cloud!?"

[dwjp]

A few people raised this feedback from training that there wasn't quite enough background on what the 'cloud' was, generally, and in the context of these lessons.

Something along the lines of explaining:

• Commodity 'on-tap' computing, i.e. in a data center, not your desk
• SaaS/PaaS/IaaS distinction
• Generic benefits (cheaper, easier, less to manage, easy to share)