[ResourcePriorities] lazyload vs. document.readyState

Hello!

Document has an attribute readyState which changes during document loading and fires a readystatechange event on it's change.

Current version of the draft doesn't mention this attribute and event, though it is strongly related to window.load event. 

My suggestion is to add a subsection about the attribute and corresponding event describing it's behavior when elements with lazyload or postpone attributes exist.

Changing readyState to complete when everything except lazyload\postpone resources is loaded seems intuitive to me, but we can also add another state to document.readyState. Whichever option is better, I believe it should be explicitly stated in the draft.

Thanks!

--
Ivan Kotenkov