Add CHANGELOG entry

Author: Shaptic

CHANGELOG.md

@@ -3,8 +3,13 @@
 
 ## Unreleased
 
-- Fix bug in asset sorting. We were not correctly sorting in ascii bytes
-  ([#606](https://github.com/stellar/js-stellar-base/pull/606))
+
+### Fix
+- Fixes a bug in asset sorting that did not correctly sort on ASCII bytes
+  ([#606](https://github.com/stellar/js-stellar-base/pull/606)).
+
+### Update
+- Migrates code to use the native `BigInt` wherever possible over the `BigNumber` dependency ([#607](https://github.com/stellar/js-stellar-base/pull/607)).
 
 
 ## [v9.0.0-beta.2](https://github.com/stellar/js-stellar-base/compare/v9.0.0-beta.1..v9.0.0-beta.2)