Added note to improve query performance with root ancestor targeting

app/imports/api/parenting/ChildSchema.js

@@ -4,6 +4,11 @@ const RefSchema = new SimpleSchema({
   id: {
     type: String,
     regEx: SimpleSchema.RegEx.Id,
+    // TODO: Rather than indexing this field, index `ancestors.0.id` to only
+    // index the root of the ancestor heirarchy to significantly reduce
+    // index size and improve performance
+    // All queries on an ancestor document need to target `ancestors.0.id` first
+    // before targeting a younger ancestor
     index: 1
   },
   collection: {