Add the Data event to the contracts readme

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](https://github.com/iotexproject/iott-dapp-example/blob/main/contracts/README.md)?

`* Data - emitted by addData``

BTW, regarding the `addData()` function:
```solidity
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)?