Add little exlanation about various classes

Author: umbynos

README.md

@@ -5,7 +5,7 @@ The supported boards are MKRGSM, MKR1000 (Wifi101) and MKR 1010 (WifiNINA).
 
 ## Arduino IoT Cloud
 
-Arduino IoT Cloud is a service which allows connecting the cloud to the world around you.
+[Arduino IoT Cloud](https://create.arduino.cc/iot/things) is a service which allows connecting the cloud to the world around you. [Here](https://www.arduino.cc/en/IoT/HomePage) an introduction.
 Now the platform is in public beta, feedbacks are welcome.
 
 
@@ -20,3 +20,20 @@ Now the platform is in public beta, feedbacks are welcome.
 - **Events**: When something happens the Arduino IoT Cloud is aware of it thanks to  application messages. For example, it might be informed by a face-recognition application that someone is at a door, or it has received a request from another app that a light be turned on.
 
 - **Software**: It's quickly autogenerated automatically by the Arduino IoT Cloud when setting up a new thing: in this way everything that you need it's almost done and ready. The connection to the cloud it's already made possible by this library. You have only to implement the callbacks linked to the properties.
+
+## ArduinoIoTCloud library
+
+The library is divided in various classes:
+- `ConnectionManager` which is responsable for the connection to the internet and to the Arduino IoT Cloud through `WiFiConnectionManager` or `GSMConnectionManager`. The selection is done using the type of board used.
+
+- `ArduinoIoTCloud` it's the leading class. It's responsable for the connection to the MQTT Broker.
+
+- `CloudSerial` it's similar to [Serial](https://www.arduino.cc/reference/en/language/functions/communication/serial/), but it's used in combination with the cloud.
+
+### ConnectionManager
+[todo]
+
+### ArduinoIoTCloud
+[todo]
+### CloudSerial
+[todo]