docs how to grab brightness

[fredgrott]

People keep asking about this, so my gist

https://gist.github.com/fredgrott/da2492e89fbb7a59e4146320be7006be

assuming that SL your deps you need to use media query and WidgetsBinding to grab a window that way it will work even before MaterialApp is initialized.

And of course, you use a Future async paring