[webrtc-stats] Pull Request: Design considerations section

alvestrand has just submitted a new pull request for 
https://github.com/w3c/webrtc-stats:

== Design considerations section ==
Adds design guidance on adding stats, on stats values when not 
relevant,
and on stats lifetimes.

Closes #20

Note: #20 says that we need to note that zero is a valid value for 
jitter,
so can't be used as an initial value for "not set yet". This isn't 
added.

See https://github.com/w3c/webrtc-stats/pull/68