add "it's OK to be odd" rule

communications_protocol.asciidoc

@@ -28,6 +28,15 @@ A Lightning Message falls into one of the following groups:
 | Custom          | 32768~65535 | Experimental or custom messages
 |=======
 
+Lightning Messages follows two forethoughtful rules: "it's OK to be odd" rule and "ignore additional data" rule. Both of them allow for future extension of Lightning Messages.
+
+=== "It's OK to be odd" rule
+
+Lightning Message type follows the _it's ok to be odd_ rule. A node is allowed to send a message of an _odd_-numbered type, but not _even_-numbered type without prior negotiation. Once a node receives a _oddly_-but-unknown-typed message, it should ignore it. However, if it receives a _evenly_-but-unknown-typed message, it is expected to fail the channels.
+
+=== "Ignore additional data" rule
+
+
 === Opening a Payment channel
 
 === forwarding, making and receiving a payment