Define best practices for monitoring of single page apps via UT / custom Entries

[igrigorik]

Continuation of w3c/charter-webperf#27.

As we iterate on L3, we should define the recommended best practices for frameworks and SPA's on how to use User Timing and custom Performance Entries to surface perf data in an interoperable way — e.g. such that various DevTool clients can consume and visualize it, RUM analytics, etc.

[yoavweiss]

Hopefully most of this will be covered with whatever we come up with for w3c/performance-timeline#168