add changelog

Author: cometkim

CHANGELOG.md

@@ -12,6 +12,10 @@
 
 # 12.0.0-alpha.5 (Unreleased)
 
+#### :rocket: New Feature
+
+- Introduce "Unified operators" for arithmetic operators (`+`, `-`, `*`, `/`, `mod`). See https://github.com/rescript-lang/rescript-compiler/pull/7057
+
 # 12.0.0-alpha.4
 
 #### :boom: Breaking Change