Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -40,6 +40,9 @@ subset of the arguments, and return a curried type with the remaining ones https
 Also, `(. int) => string => bool` is not equivalen to `(. int, string) => bool` anymore.
 These are only breaking changes for unformatted code.
 - Exponentiation operator `**` is now right-associative. `2. ** 3. ** 2.` now compile to `Math.pow(2, Math.pow(3, 2))` and not anymore `Math.pow(Math.pow(2, 3), 2)`. Parentheses can be used to change precedence.
+- `genType`: streamline the treatment of optionals as undefined https://github.com/rescript-lang/rescript-compiler/pull/6022
+  - Represent `option<t>` as `undefined | t` instead of `null | undefined | t`. This is more permissive when importing functions taking optional values (allows to use option types), but stricter when e.g. exporting ReScript functions taking arguments of option type. Fallback: use `Js.undefined<_>` instead.
+  - Represent `{x:option<string>}` as `{x:(undefined | string)}` instead of `{x?: string}`. This is more in line with TS's behaviour. Fallback: use `{x?:string}`.
 
 #### :bug: Bug Fix