(fix) note explaining identity verification for documents

0002-binary-protocol.md

@@ -127,6 +127,8 @@ Usernames can be used for only one identity registered on the blockchain.
 
 > It will maybe be possible in the future to use a username not used for a long time.
 
+> As explained below, each document contains issuers raw public keys and signatures. We can hash provided public keys to find associated identities.
+
 # 3. Documents
 
 ## 3.1. Document structure