add changelog entry for XDR serialization methods

CHANGELOG.md

@@ -6,6 +6,18 @@ A breaking change will get clearly marked in this log.
 
 ## Unreleased
 
+### Added
+- `contract.AssembledTransaction` now has a `toXDR` and `fromXDR` method for serializing the
+transaction to and from XDR. Additionally, `contract.Client` now has a `txFromXDR`. These methods
+should be used in place of `AssembledTransaction.toJSON`, `AssembledTransaction.fromJSON`, and
+`Client.txFromJSON` for multi-auth signing. The JSON methods are now deprecated. **Note you must now
+call `simulate` on the transaction before the final `signAndSend` call after all required signatures
+are gathered when using the XDR methods.
+
+### Deprecated
+- In `contract.AssembledTransaction`, `toJSON` and `fromJSON` should be replaced with `toXDR` and
+`fromXDR`. Similarly, in `contract.Client`, `txFromJSON` should be replaced with `txFromXDR`.
+
 
 ## [v12.0.1](https://github.com/stellar/js-stellar-sdk/compare/v11.3.0...v12.0.1)