[web-animations-1] Include a non-normative summary of states and transitions #2047

From @birtles on April 24, 2015 1:7

As outlined here:
https://lists.w3.org/Archives/Public/public-fx/2015AprJun/0013.html
And suggested here:
https://lists.w3.org/Archives/Public/public-fx/2015AprJun/0031.html

Copied from original issue: w3c/web-animations#92