Update to README

[samholmes]

• Add section describing signal runtime properties
  • hastiness of signal compute function (signal compute functions are invoked immediate to establish bindings to sources)
  • Equality of values optimization (signals only run with sources change ===)
• Add section describing circuits
  • circuits are signals embedded in other signals and grouped together with the same life-cycle
  • Calling off on a circuit will cleanup the embedded signals
  • A circuit is a signal, but may contain other signals itself
  • Propose naming convention to custom circuits (user-land API): prefix with “on” (e.g. onArray, onSocket, etc).
  • Explain how circuits are a higher order concept and don’t necessarily have the same function signature as a Signal type.