[webrtc-stats] add a graph showing the relationship of stats to the spec

fippo has just created a new issue for https://github.com/w3c/webrtc-stats:

== add a graph showing the relationship of stats to the spec ==
@henbos has done a whiteboard drawing of the current stats graph which I find *very* useful:
https://photos.app.goo.gl/6P0i4AV1E3szctTl1

Would be great to add this to the spec too as it explains some of the concepts much better than two pages of text could

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/348 using your GitHub account