Add CHANGELOG entry for big op gset_to_gmap lemma

CHANGELOG.md

@@ -5,6 +5,11 @@ lemma.
 
 ## Iris master
 
+**Changes in `algebra`:**
+* Add lemmas `big_opS_gset_to_gmap`, `big_opS_gset_to_gmap_L` which rewrite
+  between `gset_to_gmap` and big set ops of singleton maps.  (by Isaac van
+  Bakel)
+
 **Changes in `proofmode`:**
 
 * Remove the `*` specialization pattern. This pattern has been deprecated and a