Markhoeber/documentation/stud 1260

See draft documentation at:  http://draft-nested-components.readthedocs.org/en/latest/organizing_course.html#nested-components
Changing the group configuration might mean adding new groups, but it also might mean deleting groups, correct? If so, we should change this to something like "You can add or remove groups in your content experiment. When you add a group, you must add at least one component to the new group. When you remove a group, (this happens)." If we only want people to add groups, we should say that explicitly.