Brightness: Add some documentation on how to handle it.

[r-dent]

Currently there is no way to get the brightness from a device. Add some advice/tipps for a good approach to handle this.

For example:

If you want, you can set some sensible initial brightness in your app, save that value, and provide a slider for users to tune it further. There is no guarantee that the brightness will match your saved value after your app was in the background. But you can just apply your saved value when entering foreground. The worst that could happen, is that the brightness of the deck changes when users switch between different apps (if other apps handle it the same way).