Add info about enabling open response assessments feature

@lamagnifica @Lyla-Fischer Please review - I'd like to merge this relatively soon so I can build the docs before today's hackathon demo.
Ordinarily I would, but this is actually going to change in the next release ("Peer Assessment" will be an option on the Advanced tab when the user creates a new Problem component). We just wanted updated documentation for today's demo. Great suggestion though. :smile: