Add explainer to expose client info on "PerformanceResourceTiming" API

[monica-ch]

Explainer doc for the feedback.

[noamr]

Thanks for working on this! I'm a bit confused about how the solution (specifically option 1) maps to the use case.
Seems like the use case is about mapping resource timing entries to fetch events, but the solution in option 1 only deals with navigations and maps the clientId, which would be the same across resources?

This OP alludes to that but this explainer doesn't...

Another issue with this is that mapping a response to a clientId only works if you don't use caches, which often beats the purpose of using a service-worker in the first place. A Cache allows you to reuse responses across clients, and this kind of mapping would break on the first response reuse.

[fabiorocha]

Looks like both files are almost identical. I assume we only need one of them?