Add docs for Serverless target (#6070)

I need feedback on if this is clear enough for users.

Author: timneutkens

packages/next/README.md

@@ -1587,6 +1587,66 @@ Note: `NODE_ENV` is properly configured by the `next` subcommands, if absent, to
 
 Note: we recommend putting `.next`, or your [custom dist folder](https://github.com/zeit/next.js#custom-configuration), in `.gitignore` or `.npmignore`. Otherwise, use `files` or `now.files` to opt-into a whitelist of files you want to deploy, excluding `.next` or your custom dist folder.
 
+### Serverless deployment
+
+<details>
+  <summary><b>Examples</b></summary>
+  <ul>
+    <li><a href="https://github.com/zeit/now-examples/tree/master/nextjs">Now.sh</a></li>
+    <li>We encourage contributing more examples to this section</li>
+  </ul>
+</details>
+
+Serverless deployment dramatically improves reliability and scalability by splitting your application into many entrypoints. In case of Next.js an entrypoint is a page in the `pages` directory.
+
+To enable building serverless functions you have to enable the `serverless` build `target` in `next.config.js`:
+
+```js
+// next.config.js
+module.exports = {
+  target: 'serverless'
+}
+```
+
+The serverless target will output a single file per page, this file is completely standalone and doesn't require any dependencies to run:
+
+- `pages/index.js` => `.next/serverless/pages/index.js`
+- `pages/about.js` => `.next/serverless/pages/about.js`
+
+The signature of the Next.js Serverless function is similar to the Node.js HTTP server callback:
+
+```ts
+export function render(req: http.IncomingMessage, res: http.ServerResponse) => void
+```
+
+- [http.IncomingMessage](https://nodejs.org/api/http.html#http_class_http_incomingmessage)
+- [http.ServerResponse](https://nodejs.org/api/http.html#http_class_http_serverresponse)
+- `void` refers to the function not having a return value. Calling the function will finish the request.
+
+Next.js provides low-level APIs for Serverless as hosting platforms have different function signatures. In general you will want to wrap the output of a Next.js Serverless build with a compatability layer.
+
+For example if the platform supports the Node.js [`http.Server`](https://nodejs.org/api/http.html#http_class_http_server) class:
+
+```js
+const http = require('http')
+const page = require('./.next/serverless/about.js')
+const server = new http.Server((req, res) => page.render(req, res))
+server.listen(3000, () => console.log('Listening on http://localhost:3000'))
+```
+
+For specific platform examples see [the examples section above](#serverless-deployment).
+
+To summarize:
+
+- Low-level API for implementing Serverless deployment
+- Every page in the `pages` directory becomes a serverless function
+- Creates the smallest possible Serverless function (50Kb base zip size)
+- Optimized for fast cold start of the function
+- The serverless function has 0 dependencies (they are included in the function bundle)
+- Uses the [http.IncomingMessage](https://nodejs.org/api/http.html#http_class_http_incomingmessage) and [http.ServerResponse](https://nodejs.org/api/http.html#http_class_http_serverresponse) from Node.js
+- opt-in using `target: 'serverless'` in `next.config.js`
+- Does not load `next.config.js` when executing the function, note that this means `publicRuntimeConfig` / `serverRuntimeConfig` are not supported
+
 ## Browser support
 
 Next.js supports IE11 and all modern browsers out of the box using [`@babel/preset-env`](https://new.babeljs.io/docs/en/next/babel-preset-env.html). In order to support IE11 Next.js adds a global `Promise` polyfill. In cases where your own code or any external NPM dependencies you are using requires features not supported by your target browsers you will need to implement polyfills.