Add changelog entry for specialised Enum Int64/Word64 instances

libraries/base/changelog.md

@@ -2,6 +2,8 @@
 
 ## 4.20.0.0 *TBA*
   * Add a `RULE` to `Prelude.lookup`, allowing it to participate in list fusion ([CLC proposal #174](https://github.com/haskell/core-libraries-committee/issues/175))
+  * The `Enum Int64` and `Enum Word64` instances now use native operations on 32-bit platforms, increasing performance by up to 1.5x on i386 and up to 5.6x with the JavaScript backend. ([CLC proposal #187](https://github.com/haskell/core-libraries-committee/issues/187))
+
 
 ## 4.19.0.0 *TBA*
   * Add `{-# WARNING in "x-partial" #-}` to `Data.List.{head,tail}`.