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.
comma splice issues now. Maybe just "...you enable the Open Response Assessments feature by adding a policy key value in the course advanced settings and then edit the XML code in a Problem component." 
Or break it into two pieces?: To add a peer assessment problem to your course, you must first enable the Open Response Assessments feature by adding a policy key value to the course advanced settings. You can then edit the XML code in a Problem component to create the Open Response Assessment, as you do to create other problems.