Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
I think it would be worth highlighting that you aren't restricted to a single auto-cohort group in this case. If you have a large 70,000 student MOOC that you want to split into smaller groups plus a 1000 student alumni student body, you could create 7 auto cohort groups and 1 manual group and assign all your alumni to the manual group.