Add example JavaScript

Author: lindboe

docs/the-new-architecture/pillars-turbomodule.md

@@ -657,3 +657,46 @@ Then, to manually link your new TurboModule:
             return rncore_ModuleProvider(moduleName, params);
         }
         ```
+
+### JavaScript
+
+Now you can use your TurboModule calculator in your app!
+
+Here's an example App.js file using the `add` method:
+
+```js title="App.js"
+/**
+ * Sample React Native App
+ * https://github.com/facebook/react-native
+ *
+ * @format
+ * @flow strict-local
+ */
+import React from 'react';
+import {useState} from 'react';
+import type {Node} from 'react';
+import {SafeAreaView, StatusBar, Text, Button} from 'react-native';
+import RTNCalculator from 'rtn-calculator/js/NativeCalculator.js';
+
+const App: () => Node = () => {
+  const [currentResult, setResult] = useState<number | null>(null);
+  return (
+    <SafeAreaView>
+      <StatusBar barStyle={'dark-content'} />
+      <Text style={{marginLeft: 20, marginTop: 20}}>
+        3+7={currentResult ?? '??'}
+      </Text>
+      <Button
+        title="Compute"
+        onPress={async () => {
+          const result = await RTNCalculator.add(3, 7);
+          setResult(result);
+        }}
+      />
+    </SafeAreaView>
+  );
+};
+export default App;
+```
+
+Try this out to see your TurboModule in action!