Update change log to indicate that the beacon is now optional

Author: sfauvel

CHANGELOG.md

@@ -25,6 +25,8 @@ As a minor extension, we have adopted a slightly different versioning convention
 
 - Add prettier configuration to standardize the code formatting in the repository.
 
+- Field `beacon` become optional in `CertificatePendingMessage` response of `/certificate-pending` route.
+
 - **UNSTABLE** Cardano transactions certification:
 
   - Optimize the performances of the computation of the proof with a Merkle map.