Updated proposal for aggregate measurement

Hey everyone,
I wanted to bring to the group's attention a few documents we just
published in the conversion measurement WICG repo:

   - SERVICE.md
   <https://github.com/WICG/conversion-measurement-api/blob/master/SERVICE.md>is
   an outline of a generic service architecture that provides private
   aggregate output.
   - AGGREGATE.md
   <https://github.com/WICG/conversion-measurement-api/blob/master/AGGREGATE.md>
   is an outline of how the service could be used to achieve aggregate
   conversion measurement.

Happy to discuss this more in the upcoming meeting if folks are interested!

-Charlie