document whole-set scheduling

ai/shared.md

@@ -9,6 +9,9 @@ these rules.
 - Attainly is in early development.
 - Attainly helps users break hierarchical goals into scheduled assignments,
   complete daily work, and track progress toward a target date.
+- Schedules target whole sets. Users choose an element kind as the assignment
+  granularity. To schedule a subset, create a separate named set containing
+  that subset.
 - Schedule recalculation must preserve completed work while redistributing
   unfinished assignments across the remaining dates.
 - Planned features in `README.md` are future ideas, not authorized scope.