update CHANGELOG

CHANGELOG.md

@@ -21,6 +21,8 @@ Coq development, but not every API-breaking change is listed.  Changes marked
 * Renaming and moving things around: uPred and the rest of the base logic are
   in `base_logic`, while `program_logic` is for everything involving the
   general Iris notion of a language.
+* [#] Slightly weaker notion of atomicity: an expression is atomic if it reduces
+  in one step to something that does not reduce further.
 * Changed notation for embedding Coq assertions into Iris.  The new notation
   is ⌜φ⌝.  Also removed `=` and `⊥` from the Iris scope.
   (The old notations are provided in `base_logic.deprecated`.)