[webrtc-stats] Describe the reason for "sum & count" rather than "average"

alvestrand has just created a new issue for 
https://github.com/w3c/webrtc-stats:

== Describe the reason for "sum & count" rather than "average" ==
In the design considerations, we should include explanations of why we
 use "sum & count" rather than "average" - the reason being that 
"average" implies averaging over some fixed time interval, which may 
or may not be what the app wants; sum & count allows us to do 
getStats() twice and calculate the average between them over an 
app-chosen time interval - which may be long or short.


Please view or discuss this issue at 
https://github.com/w3c/webrtc-stats/issues/69 using your GitHub 
account