[Enhancement] 'pin' and 'gpio' are not the same thing

Thank you for providing this functionality.

It took me a while to get it running on my Raspberry Pi 3B until I figured out that in

```
gpio_modules:
  - name: raspberrypi
    module: raspberrypi
```
the 
`   pin:`
does not refer to the pin number, but to the GPIO number as in https://pinout.xyz/

Being mechanically minded, the documentation at https://mqtt-io.app did not make this clear to me.