Improve docs to clarify that for some command the arduino-cli is a pre-requisite

[csarnataro]

I tried to create a device in the Cloud with the command arduino-cloud-cli device create.
It turned out that, for security reasons (see this interesting blog post), this command has to configure the board via the so-called "provisioning process" which requires to flash a specific sketch on the board as part of the process.
For this reason, internally, it tries to call the arduino-cli to flash such a sketch on the board.
Since I didn't have the arduino-cli installed, the process failed.
After installing it, the process completed successfully.

I would suggest to update the README file to add this dependency from the arduino-cli for some commands (namely, the device create command).