doc: add examples for implementing ESM

doc/api/esm.md

@@ -13,6 +13,27 @@ ECMAScript modules are [the official standard format][] to package JavaScript
 code for reuse. Modules are defined using a variety of [`import`][] and
 [`export`][] statements.
 
+The following example of an ES module exports a function:
+
+```js
+// addTwo.js
+function addTwo(num) {
+  return num + 2;
+}
+
+export { addTwo };
+```
+
+The following example of an ES module imports the function from `addTwo.js`:
+
+```js
+// app.js
+import { addTwo } from './addTwo.js';
+
+// Prints: 6
+console.log(addTwo(4));
+```
+
 Node.js fully supports ECMAScript modules as they are currently specified and
 provides limited interoperability between them and the existing module format,
 [CommonJS][].