Document adding a router (facebook#3709)

Author: gaearon

packages/react-scripts/template/README.md

@@ -34,6 +34,7 @@ You can find the most recent version of this guide [here](https://github.com/fac
 - [Adding Bootstrap](#adding-bootstrap)
   - [Using a Custom Theme](#using-a-custom-theme)
 - [Adding Flow](#adding-flow)
+- [Adding a Router](#adding-a-router)
 - [Adding Custom Environment Variables](#adding-custom-environment-variables)
   - [Referencing Environment Variables in the HTML](#referencing-environment-variables-in-the-html)
   - [Adding Temporary Environment Variables In Your Shell](#adding-temporary-environment-variables-in-your-shell)
@@ -804,6 +805,26 @@ In the future we plan to integrate it into Create React App even more closely.
 
 To learn more about Flow, check out [its documentation](https://flowtype.org/).
 
+## Adding a Router
+
+Create React App doesn't prescribe a specific routing solution, but [React Router](https://reacttraining.com/react-router/) is the most popular one.
+
+To add it, run:
+
+```sh
+npm install --save react-router react-router-dom
+```
+
+Alternatively you may use `yarn`:
+
+```sh
+yarn add react-router react-router-dom
+```
+
+To try it, delete all the code in `src/App.js` and replace it with any of the examples on its website. The [Basic Example](https://reacttraining.com/react-router/web/example/basic) is a good place to get started.
+
+Note that [you may need to configure your production server to support client-side routing](#serving-apps-with-client-side-routing) before deploying your app.
+
 ## Adding Custom Environment Variables
 
 >Note: this feature is available with `[email protected]` and higher.