Clarify how to change system UI on insets library

[pablisco]

Is your feature request related to a problem? Please describe.
The Guide on here mentions that we need to set setDecorFitsSystemWindows to false. However, there is no mention about how to make the status bar and navigation bars to be translucent.

Describe the solution you'd like
I've managed to make it work by adding android:statusBarColor and android:navigationBarColor to my xml Theme. However, I then realised that there is a separate library we can use to control the system UI. It may be worth mentioning it on the inset lib for future readers :)

PS: Thank you for the great work 😃