Update next-bundle-analyzer readme (#14976)

https://github.com/vercel/next.js/issues/14969

It didn't specify on how to include the config with function config. This PR clarify the way to use the config with function. 

packages/next-bundle-analyzer/readme.md

@@ -25,6 +25,14 @@ const withBundleAnalyzer = require('@next/bundle-analyzer')({
 module.exports = withBundleAnalyzer({})
 ```
 
+Or configuration as a function:
+
+```js
+module.exports = (phase, defaultConfig) => {
+  return withBundleAnalyzer(defaultConfig)
+}
+```
+
 Then you can run the command below:
 
 ```bash