add a comment to pinpoint the new, verge-related fields

Co-authored-by: Mikhail Kalinin <[email protected]>

Author: gballet

specs/verge/beacon-chain.md

@@ -79,7 +79,7 @@ class ExecutionPayload(Container):
     block_hash: Hash32  # Hash of execution block
     # Extra payload field
     transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]
-    execution_witness: ExecutionWitness
+    execution_witness: ExecutionWitness  # [New in Verge]
 ```
 
 #### `ExecutionPayloadHeader`