Updated instructions for open weather API 3.0

I'm getting 401 errors in the browser console saying this:
```
MMM-WeatherChart.js:119 Error: MMM-WeatherChart: apiKey is invalid
```

Do we need to sign up for a separate API to get access now? 
https://openweathermap.org/api/one-call-3

If so, can you update the readme to mention this? 