Updated Output node feature definitions (markdown)

Output-node-feature-definitions.md

@@ -6,6 +6,7 @@ This page defines the features and design conventions that each output node shou
 
 ### Output nodes SHOULD:
 * Report errors using <code>node.warn()</code> or <code>node.error()</code> if the external systems are in an unexpected state/behaviour
+* Clean up their state upon close <code>node.on("close", function() {}</code>
 
 ### Output nodes COULD:
 * TBC