Request: Add more detailed examples.

Hi,

Would it be possible to have an example of how to create a plugin which, connected to a topic, takes the message (payload), transforms it and publishes it on another topic?

Example :
```
topic test/hello/msg
msg {"val1" : 50, "val2": 30}
Function to app : sum(val1, val2)
publish on topic test/hello/result -> {"val1" : 50, "val2": 30, "sum": 80}
```

A thousand thanks.