Missing section: How to send a command

[manucarrace]

Although there is a section in the device API explaining how to act upon a device, it should be nice to have a section (in Data API or in Topics) explaining how to send a command from Context Broker, which is the way the platform provides to send commnads.

It would be great @dmoranj feedback.

[dmoranj]

I'll update the documentation adding a section about commands (explaining how they are declared and used).