Permit PADDING in Initial. Closes #1645.

draft-ietf-quic-transport.md

@@ -702,8 +702,9 @@ server may send multiple Initial packets.  The cryptographic key exchange could
 require multiple round trips or retransmissions of this data.
 
 The payload of an Initial packet includes a CRYPTO frame (or frames) containing
-a cryptographic handshake message, ACK frames, or both. The first CRYPTO frame
-sent always begins at an offset of 0 (see {{handshake}}).
+a cryptographic handshake message, ACK frames, or both.  PADDING frames are also
+permitted.  The first CRYPTO frame sent always begins at an offset of 0 (see
+{{handshake}}).
 
 The first packet sent by a client always includes a CRYPTO frame that contains
 the entirety of the first cryptographic handshake message.  This packet, and the