Add changelog entry

Author: GnomedDev

CHANGELOG.md

@@ -49,6 +49,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `get_index` and `get_index_mut` to `IndexMap`.
 - Added `String::uDisplay`.
 - Added `LenT` generic to `Vec<T, N>` and `VecView<T>` to save memory when using a sane capacity value.
+- Migrated `Idx` generic for `SortedLinkedList` to use the new `LenType` trait, allowing for `Idx` inference.
 
 ### Changed