add page about smart collections

content/howto/smart-collections.md

@@ -0,0 +1,12 @@
+---
+title: create a "smart collection"
+id: luminosity-masks
+weight: 120
+draft: false
+author: "people"
+---
+
+Every [_collection_](../module-reference/utility-modules/shared/collections.md)
+in darktable is "smart", and will immediately update when an image's
+metadata changes such that it is newly included in or excluded from
+the collection's search criteria.