Add the Data event to the contracts readme

[simonerom]

It seems GitHub forking is down today and I can't make the change myself:
can you add the following at line 100 to this README?

`* Data - emitted by addData``

BTW, regarding the addData() function:

function addData(
        string memory imei,
        uint32 _type,
        bytes memory data,
        uint32 timestamp,
        bytes memory signature,
        uint256 gas
    )

What is _type for (where is it defined)? And what exactly timestamp refers to (considering that we already have the timestamp in the data sent by the device)?