docs: update changelog

Author: glennsl

CHANGELOG.md

@@ -25,6 +25,7 @@
 - Add `panic`/`Error.panic`. https://github.com/rescript-association/rescript-core/pull/72
 - The globally available `null` value now originates from `Nullable` and not `Null`, just like the globally available `undefined` value does. https://github.com/rescript-association/rescript-core/pull/88
 - Add `Int.range` and `Int.rangeWithOptions`, https://github.com/rescript-association/rescript-core/pull/52
+- Add `flat`, `expect`, and `or` to `Option`, deprecate `orElse`. https://github.com/rescript-association/rescript-core/pull/57
 
 ### Documentation