[webrtc-priority] new commits pushed by alvestrand

The following commits were just pushed by alvestrand to https://github.com/w3c/webrtc-priority:

* Add a first-principles explainer to doc

This was requested from TAG, but I can't find where it was requested at the moment. @dontcallmedom may know.
  by Harald Alvestrand
https://github.com/w3c/webrtc-priority/commit/1f26dd52f8511fe274d412e9a4711aa661c36773