README should note that iPhone simulators cannot access BLE devices

[allendav]

Optional Step 7 implies that testing with a real BBPOS Chipper 2X BT device is as simple as changing a boolean. It would probably be helpful to also note that you must also then switch to testing with a real iOS device. The simulator bundled with Xcode does not support BLE.

[allendav]

May also want to mention: stripe/example-terminal-backend#30

and that they'll also need to update the baseURL in APIClient.swift to the IP of whatever their Sinatra server is running on, e.g.:

static let baseURL = URL(string: "http://10.0.0.161:4567")