Updated Output node feature definitions (markdown)

Output-node-feature-definitions.md

@@ -1,5 +1,18 @@
 This page defines the features and design conventions that each output node should adhere to. The page is currently under development and changes to its contents are possible. Once the behaviour is agreed upon, these conventions are going to be migrated onto the Node-RED website for easy reference.
 
-### Output nodes MUST:
+### Putput nodes MUST:
 
-CONSIDER OUTPUT NODES and populate this page
+* Produce a clearly and well defined behaviour that affects some externals outside Node-RED
+
+### Input nodes SHOULD:
+* Report errors using <code>node.warn()</code> or <code>node.error()</code> if the external systems are in an unexpected state/behaviour
+
+### Input nodes COULD:
+* Poll at regular intervals to see if msg trigger conditions are met:
+<code>
+                node.interval = setInterval(function() {
+                    node.emit("input", {});
+                }, REPEAT_IN_MILLIS);
+</code>
+* Alternatively they could fire by being triggered by callbacks =>  TODO define behaviour here
+* Reserve the right not to send anything at all, ever if conditions are never met