Document what npm build does and pushState (facebook#933)

* Document what npm build does and pushState

* Fix typos, add express example

* Tweaks

* Update README.md

Author: ldct

packages/react-scripts/template/README.md

@@ -47,6 +47,7 @@ You can find the most recent version of this guide [here](https://github.com/fac
 - [Developing Components in Isolation](#developing-components-in-isolation)
 - [Making a Progressive Web App](#making-a-progressive-web-app)
 - [Deployment](#deployment)
+  - [Serving Apps with Client-Side Routing](#serving-apps-with-client-side-routing)
   - [Building for Relative Paths](#building-for-relative-paths)
   - [GitHub Pages](#github-pages)
   - [Heroku](#heroku)
@@ -143,6 +144,8 @@ It correctly bundles React in production mode and optimizes the build for the be
 The build is minified and the filenames include the hashes.<br>
 Your app is ready to be deployed!
 
+See the section about [deployment](#deployment) for more information.
+
 ### `npm run eject`
 
 **Note: this is a one-way operation. Once you `eject`, you can’t go back!**
@@ -957,7 +960,51 @@ You can turn your React app into a [Progressive Web App](https://developers.goog
 
 ## Deployment
 
-## Building for Relative Paths
+`npm run build` creates a `build` directory with a production build of your app. Set up your favourite HTTP server so that a visitor to your site is served `index.html`, and requests to static paths like `/static/js/main.<hash>.js` are served with the contents of the `/static/js/main.<hash>.js` file. For example, Python contains a built-in HTTP server that can serve static files:
+
+```sh
+cd build
+python -m SimpleHTTPServer 9000
+```
+
+If you're using [Node](https://nodejs.org/) and [Express](http://expressjs.com/) as a server, it might look like this:
+
+```javascript
+const express = require('express');
+const path = require('path');
+const app = express();
+
+app.use(express.static('./build'));
+
+app.get('/', function (req, res) {
+  res.sendFile(path.join(__dirname, './build', 'index.html'));
+});
+
+app.listen(9000);
+```
+
+Create React App is not opinionated about your choice of web server. Any static file server will do. The `build` folder with static assets is the only output produced by Create React App.
+
+However this is not quite enough if you use client-side routing. Read the next section if you want to support URLs like `/todos/42` in your single-page app.
+
+### Serving Apps with Client-Side Routing
+
+If you use routers that use the HTML5 [`pushState` history API](https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries) under the hood (for example, [React Router](https://github.com/ReactTraining/react-router) with `browserHistory`), many static file servers will fail. For example, if you used React Router with a route for `/todos/42`, the development server will respond to `localhost:3000/todos/42` properly, but an Express serving a production build as above will not.
+
+This is because when there is a fresh page load for a `/todos/42`, the server looks for the file `build/todos/42` and does not find it. The server needs to be configured to respond to a request to `/todos/42` by serving `index.html`. For example, we can amend our Express example above to serve `index.html` for any unknown paths:
+
+```diff
+ app.use(express.static('./build'));
+
+-app.get('/', function (req, res) {
++app.get('/*', function (req, res) {
+   res.sendFile(path.join(__dirname, './build', 'index.html'));
+ });
+```
+
+Now requests to `/todos/42` will be handled correctly both in development and in production.
+
+### Building for Relative Paths
 
 By default, Create React App produces a build assuming your app is hosted at the server root.<br>
 To override this, specify the `homepage` in your `package.json`, for example: