Created Design: Node settings (markdown)

Design:-Node-settings.md

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+/settings API returns only the following built-in fields by default.
+
+http://nodered.jp/docs/api/admin/methods/get/settings/
+
+A node can add their own settings to the above settings.
+
+### Adding a setting
+The way to register a node is as follows (https://nodered.org/docs/creating-nodes/first-node).
+
+`RED.nodes.registerType("lower-case",LowerCaseNode);`
+
+To add a node setting, add setting object.
+```
+RED.nodes.registerType("lower-case",LowerCaseNode,{
+  settings: {
+    lowerCaseLength:{
+      value: "10",
+      exportable: true
+    }
+  }
+});
+```
+* `value` - The value corresponding with the object name.
+* `exportable` - Whether the value will be output or not.
+
+### Overwriting a node setting by settings.js