Aggregate conversion measurement explainer

Hey folks,
I added a section
<https://github.com/csharrison/conversion-measurement-api/blob/master/AGGREGATE.md>
to my conversion measurement repo that discusses a possible API that could
support aggregate-only measurement. There's nothing super concrete yet but
I attempted to survey the field of possible technologies that could be
useful here.

Feel free to take a look. Happy to discuss these ideas on the upcoming call
or in person soon!