Improve documentation for IotfDevice

It's not at all clear form the docs what needs to be passed to the constructor to create the device.

It's either a device.cfg file that looks like the one in the scripts/connectors/iotf

or a set of properties

https://docs.internetofthings.ibmcloud.com/devices/libraries/java.html

//Provide the device specific data, as well as Auth-key and token using Properties class
Properties options = new Properties();

options.setProperty("org", "uguhsp");
options.setProperty("type", "iotsample-arduino");
options.setProperty("id", "00aabbccde03");
options.setProperty("auth-method", "token");
options.setProperty("auth-token", "AUTH TOKEN FOR DEVICE");