Mention Custom App GIP in UPGRADING (vercel#8647)

This tells the user to check their custom <App> for `getInitialProps` if they previously copied the example.

This should help them leverage automatic static optimization!

UPGRADING.md

@@ -10,6 +10,33 @@ Next.js 9's dynamic routes are **automatically configured on [Now](https://zeit.
 
 You can read more about [Dynamic Routing here](https://github.com/zeit/next.js#dynamic-routing).
 
+#### Check your Custom <App> (`pages/_app.js`)
+
+If you previously copied the [Custom `<App>`](https://nextjs.org/docs#custom-app) example, you may be able to remove your `getInitialProps`.
+
+Removing `getInitialProps` from `pages/_app.js` (when possible) is important to leverage new Next.js features!
+
+The following `getInitialProps` does nothing and may be removed:
+
+```js
+class MyApp extends App {
+  // Remove me, I do nothing!
+  static async getInitialProps({ Component, ctx }) {
+    let pageProps = {}
+
+    if (Component.getInitialProps) {
+      pageProps = await Component.getInitialProps(ctx)
+    }
+
+    return { pageProps }
+  }
+
+  render() {
+    // ... etc
+  }
+}
+```
+
 ## Breaking Changes
 
 #### `@zeit/next-typescript` is no longer necessary

errors/opt-out-automatic-prerendering.md

@@ -11,7 +11,7 @@ This causes **all pages** to be executed on the server -- disabling [Automatic P
 Be sure you meant to use `getInitialProps` in `pages/_app`!
 There are some valid use cases for this, but it is often better to handle `getInitialProps` on a _per-page_ basis.
 
-If you copied the [Custom `<App>`](https://nextjs.org/docs#custom-app) example, you may be able to remove your `getInitialProps`.
+If you previously copied the [Custom `<App>`](https://nextjs.org/docs#custom-app) example, you may be able to remove your `getInitialProps`.
 
 The following `getInitialProps` does nothing and may be removed: