document 'how it works' and 'philosophy'

[sam]

Describe the following concepts in the README:

• Entrypoint, Runtime containers and Client instances - how infrastructure constructs are mapped to their runtime (or 'run-side') interface.
• Shapes - the embedded type-system and how it glues services together and hides low-level AWS protocol details.

Describe why this project is called punchcard and what it aims to demonstrate