Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -56,6 +56,7 @@ These are only breaking changes for unformatted code.
 - Treat `@meth` annotation as making the type uncurried for backwards compatibitly with some examples https://github.com/rescript-lang/rescript-compiler/pull/5845
 - Process `@set` annotation for field update as generating an uncurried function https://github.com/rescript-lang/rescript-compiler/pull/5846
 - Treat uncurried application of primitives like curried application, which produces better output https://github.com/rescript-lang/rescript-compiler/pull/5851
+- New internal representation for uncurried functions using built-in type `function$<fun_type, arity>` this avoids having to declare all the possible arities ahead of time https://github.com/rescript-lang/rescript-compiler/pull/5870
 
 # 10.1.1