Add a README

[addisonj]

We don't have one... This doesn't all need to be in the README (links to other docs would be great)

Should include:

• General idea behind ftl-engine (pluggable graph execution)
• Quick Start Example (with screencast/gif demo?)
• More fleshed out example
• Explaining graph generation (both what is included and how you could generate another way)
• Activity plugins
• Small description of how it works (SWF, S3 for claim check, etc)

Will update this list as gaps show up