Add UPGRADING doc (#7773)

UPGRADING.md

+# Migrating from v8 to v9
+
+## Breaking Changes
+
+#### `next/dynamic` no longer renders "loading..." by default while loading
+
+Dynamic components will not render anything by default while loading. You can still customize this behavior by setting the `loading` property:
+
+```jsx
+import dynamic from 'next/dynamic'
+
+const DynamicComponentWithCustomLoading = dynamic(
+  () => import('../components/hello2'),
+  {
+    loading: () => <p>Loading</p>,
+  }
+)
+```
+
+#### `withAmp` has been removed in favor of an exported configuration object
+
+Next.js now has the concept of page-level configuration, so the `withAmp` higher-order component has been removed for consistency.
+
+This change can be **automatically migrated by running the following commands in the root of your Next.js project:**
+
+```bash
+curl -L https://github.com/zeit/next-codemod/archive/master.tar.gz | tar -xz --strip=2 next-codemod-master/transforms/withamp-to-config.js npx jscodeshift -t ./withamp-to-config.js pages/**/*.js
+```
+
+To perform this migration by hand, or view what the codemod will produce, see below:
+
+**Before**
+
+```jsx
+import { withAmp } from 'next/amp'
+
+function Home() {
+  return <h1>My AMP Page</h1>
+}
+
+export default withAmp(Home)
+// or
+export default withAmp(Home, { hybrid: true })
+```
+
+**After**
+
+```jsx
+export default function Home() {
+  return <h1>My AMP Page</h1>
+}
+
+export const config = {
+  amp: true,
+  // or
+  amp: 'hybrid',
+}
+```
+
+#### `next export` no longer exports pages as `index.html`
+
+Previously, exporting `pages/about.js` would result in `out/about/index.html`. This behavior has been changed to result in `out/about.html`.
+
+You can revert to the previous behavior by creating a `next.config.js` with the following content:
+
+```js
+// next.config.js
+module.exports = {
+  exportTrailingSlash: true,
+}
+```
+
+## Deprecated Features
+
+#### `next/dynamic` has deprecated loading multiple modules at once
+
+The ability to load multiple modules at once has been deprecated in `next/dynamic` to be closer to React's implementation (`React.lazy` and `Suspense`).
+
+Updating code that relies on this behavior is relatively straightforward! We've provided an example of a before/after to help you migrate your application:
+
+**Before**
+
+```jsx
+import dynamic from 'next/dynamic'
+
+const HelloBundle = dynamic({
+  modules: () => {
+    const components = {
+      Hello1: () => import('../components/hello1').then(m => m.default),
+      Hello2: () => import('../components/hello2').then(m => m.default),
+    }
+
+    return components
+  },
+  render: (props, { Hello1, Hello2 }) => (
+    <div>
+      <h1>{props.title}</h1>
+      <Hello1 />
+      <Hello2 />
+    </div>
+  ),
+})
+
+function DynamicBundle() {
+  return <HelloBundle title="Dynamic Bundle" />
+}
+
+export default DynamicBundle
+```
+
+**After**
+
+```jsx
+import dynamic from 'next/dynamic'
+
+const Hello1 = dynamic(() => import('../components/hello1'))
+const Hello2 = dynamic(() => import('../components/hello2'))
+
+function HelloBundle({ title }) {
+  return (
+    <div>
+      <h1>{title}</h1>
+      <Hello1 />
+      <Hello2 />
+    </div>
+  )
+}
+
+function DynamicBundle() {
+  return <HelloBundle title="Dynamic Bundle" />
+}
+
+export default DynamicBundle
+```