Update README with instructions about dependency from 'arduino-cli' (#137)

* Update README with instructions about the dependency from the 'arduino-cli' to create a device

* Fixed URL of Arduino CLI homepage

* Fixed URL of Arduino CLI installation instruction

Author: csarnataro

Diff for: README.md

@@ -10,6 +10,9 @@ This code is licensed under the terms of the GNU Affero General Public License v
 
 * Install [the latest release](https://github.com/arduino/arduino-cloud-cli/releases).
 * A client ID and a secret ID, retrievable from the [cloud](https://cloud.arduino.cc/home/api-keys) by creating a new API key.
+* Optional: the command `arduino-cloud-cli device create` requires the `arduino-cli` tool installed on the system. 
+  So, if you plan to create a device on the Arduino Cloud via CLI, you need to install the `arduino-cli` following the instructions at [https://arduino.github.io/arduino-cli/latest/installation](https://arduino.github.io/arduino-cli/latest/installation)
+
 
 ## Usage