[screen-wake-lock] Add a state transition diagram (#325)

marcoscaceres has just created a new issue for https://github.com/w3c/screen-wake-lock:

== Add a state transition diagram ==
In https://github.com/w3c/screen-wake-lock/issues/282, @David-Chadwick suggested:  

> A state transition diagram would be extremely beneficial to understand when the lock is on or off and how it is activated/deactived etc.

We did something similar for Payment Request, it I personally found it quite helpful while testing:  
https://www.w3.org/TR/payment-request/#dfn-state

Anyone keep to help create one?  

Please view or discuss this issue at https://github.com/w3c/screen-wake-lock/issues/325 using your GitHub account


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