docs(core): how and when to use delegate-focus-mixin.js about aybolit HOT 3 OPEN

The main reason is that only Chrome currently supports the sequential focus navigation:

this.attachShadow({mode: 'open', delegatesFocus: true});

The mixin is needed whenever you have a focusable element (input, button etc) within a shadow root. https://github.com/vaadin/vaadin-control-state-mixin does pretty much the same thing.

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Maybe a docs(...): ... commit would be a good idea about this in some README.md.

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The mixin is considered implementation detail at the moment. But yes, we could add some docs.

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