Merge pull request #295 from mxinden/plaintext-prefix

plaintext: Clarify only handshake messages being framed

plaintext/README.md

@@ -112,9 +112,11 @@ section of the connection establishment spec][conn-spec-protocol-negotiation].
 
 ### Message Framing
 
-All messages sent over the wire are prefixed with the message length in bytes,
-encoded as an unsigned variable length integer as defined by the [multiformats
-unsigned-varint spec][uvarint-spec].
+All [handshake messages](#messages) sent over the wire are prefixed with the
+message length in bytes, encoded as an unsigned variable length integer as
+defined by the [multiformats unsigned-varint spec][uvarint-spec]. Actual
+payloads exchanged once the plaintext handshake has completed are NOT prefixed
+with their lengths, but sent as-is.
 
 ### Exchange