Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -31,6 +31,7 @@ The `make` function of components is generated as an uncurried function.
 Use best effort to determine the config when formatting a file.
 https://github.com/rescript-lang/rescript-compiler/pull/5968 https://github.com/rescript-lang/rescript-compiler/pull/6080 https://github.com/rescript-lang/rescript-compiler/pull/6086 https://github.com/rescript-lang/rescript-compiler/pull/6087
 - Customization of runtime representation of variants. This is work in progress. E.g. some restrictions on the input. See comments of the form "TODO: put restriction on the variant definitions allowed, to make sure this never happens". https://github.com/rescript-lang/rescript-compiler/pull/6095
+- Introduce untagged variants https://github.com/rescript-lang/rescript-compiler/pull/6103
 
 #### :boom: Breaking Change