Adds sorted hash as event Ids.

01.md

@@ -42,6 +42,8 @@ To obtain the `event.id`, we `sha256` the serialized event. The serialization is
 ]
 ```
 
+Once the `sha256` is computed, replace the first 4 bytes of the hash byte array by the 4-byte, signed, little-endian representation of the `created_at`. When verifying the event, Clients MUST check if the `id` matches the full hash OR the sorted hash.
+
 To prevent implementation differences from creating a different event ID for the same event, the following rules MUST be followed while serializing:
 - UTF-8 should be used for encoding.
 - Whitespace, line breaks or other unnecessary formatting should not be included in the output JSON.