[performance-timeline] async timeline + events

Hey all, couple of quick notes based on discussion at TPAC:

1) Suggested text to clarify that timeline is async:
https://github.com/w3c/performance-timeline/commit/b2e18e2f36e3e2755341c9dd005e545785d8e882

Any comments or objections before I merge it?

2) Re, event notifications: consensus was that the use case is good,
assuming we can spec it in a reasonable + performant way (e.g. per-event
notifications may be too costly)... However, we didn't come up with any
concrete proposals for how to do so. See [1], [2] for background.
Suggestions welcome! :)

ig

[1]
http://lists.w3.org/Archives/Public/public-web-perf/2014Oct/thread.html#msg97
[2] https://github.com/w3c/performance-timeline/issues/1