README: document temperature and ABC readings

function uart_callback(data)

	if mh_z19.parse_frame(data) then

		-- mh_z19.co2 contains the CO₂ concentration in ppm

		-- mh_z19.co2       : CO₂ concentration [ppm]

		-- mh_z19.temp      : device temperature [°c]

		-- mh_z19.abc_ticks : ticks since last Automatic Baseline Correction

		-- mh_z19.abc_count : number of Automatic Baseline Corrections performed since power-on

	end

end

port:write(mhz19.c_query)

```

The sensor performs a CO₂ measurement every one to five seconds, depending on hardware revision.

## Application Example

**init.lua** is an example application with HomeAssistant integration.