Created Input node feature definitions (markdown)

Input-node-feature-definitions.md

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+This page defines the features and design conventions that each input 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.
+
+Input nodes MUST:
+
+* Create a new msg JavaScript object for every distinctive output they wish to generate. This is to be achieved for example by:
+<code>
+                node.on("input", function(msg) {
+                    msg = {};
+                    handleInputNodeInput(node, msg);
+                });
+</code>