weather: check for connection, improve README

[rigrig]

The weather app doesn't check whether Gadgetbridge is connected or not, just that it has not received weather data, and if not it shows 'Is Gadgetbridge connected?'. This is confusing, especially for new users who just set up Gadgedbridge correctly and are only missing the weather.

And the readme should be more explicit about the two steps needed to get it working:

1. Gadgetbridge
2. Weather reporting