Add README to custom node example

Author: harbournick

examples/vue-custom-node-example/README.md

@@ -0,0 +1,33 @@
+# Custom Nodes in SuperDoc
+
+You can create custom nodes (or extensions/plugins). For this example we'll create a simple custom node and pass it to the editor.
+
+First, [we've created a basic example of a custom node here](https://github.com/Harbour-Enterprises/SuperDoc/blob/main/examples/vue-custom-node-example/src/plugins/MyCustomNodePlugin.js) showing __some__ of the API we have access to.
+
+We simply import it into our document editor, using:
+```
+import { myCustomNode } from '../plugins/MyCustomNodePlugin';
+```
+
+And pass it to the editor via our config:
+```
+const config = {
+  // Our config...
+  editorExtensions: [myCustomNode],
+}
+```
+
+And that's it! Our editor now can use our custom node.
+
+## Custom commands
+Since our custom node creates a custom command `insertCustomNode` that takes an **object** as a param with keys `content` and `id`, we can use it via:
+```
+activeEditor.commands.insertCustomNode({ content, id })
+```
+
+## Running the example
+```
+npm install && npm run dev
+```
+
+The example inserts two instances of our custom node - once using a generic editor `insertContent` command and some HTML, and the second time using our newly-created custom command `insertCustomNode`