Update CHANGELOG.md

Author: cannorin

CHANGELOG.md

@@ -23,6 +23,8 @@
 - Initial support for JSX V4 including genType, still work in progress.
   - :boom: when V4 is activated, at most one component is allowed for each module.
 
+- Add placeholder types for ES6 collections (`Set`, `Map`, `WeakSet`, `WeakMap`) https://github.com/rescript-lang/rescript-compiler/pull/5630
+
 #### :bug: Bug Fix
 
 - Fix issue where pipe `->` processing eats up attributes https://github.com/rescript-lang/rescript-compiler/pull/5581