Adding link to “Customizing” create-react-app (facebook#1121)

Add documentation for customizing Bootstrap theme

packages/react-scripts/template/README.md

@@ -30,6 +30,7 @@ You can find the most recent version of this guide [here](https://github.com/fac
   - [When to Use the `public` Folder](#when-to-use-the-public-folder)
 - [Using Global Variables](#using-global-variables)
 - [Adding Bootstrap](#adding-bootstrap)
+  - [Using a Custom Theme](#using-a-custom-theme)
 - [Adding Flow](#adding-flow)
 - [Adding Custom Environment Variables](#adding-custom-environment-variables)
   - [Referencing Environment Variables in the HTML](#referencing-environment-variables-in-the-html)
@@ -585,7 +586,7 @@ Alternatively, you can force the linter to ignore any line by adding `// eslint-
 
 You don’t have to use [React Bootstrap](https://react-bootstrap.github.io) together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:
 
-Install React Bootstrap and Bootstrap from NPM. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:
+Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:
 
 ```
 npm install react-bootstrap --save
@@ -609,6 +610,17 @@ import { Navbar, Jumbotron, Button } from 'react-bootstrap';
 
 Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example [`App.js`](https://gist.githubusercontent.com/gaearon/85d8c067f6af1e56277c82d19fd4da7b/raw/6158dd991b67284e9fc8d70b9d973efe87659d72/App.js) redone using React Bootstrap.
 
+### Using a Custom Theme
+
+Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br>
+We suggest the following approach:
+
+* Create a new package that depends on the package you wish to customize, e.g. Bootstrap.
+* Add the necessary build steps to tweak the theme, and publish your package on npm.
+* Install your own theme npm package as a dependency of your app.
+
+Here is an example of adding a [customized Bootstrap](https://medium.com/@tacomanator/customizing-create-react-app-aa9ffb88165) that follows these steps.
+
 ## Adding Flow
 
 Flow is a static type checker that helps you write code with fewer bugs. Check out this [introduction to using static types in JavaScript](https://medium.com/@preethikasireddy/why-use-static-types-in-javascript-part-1-8382da1e0adb) if you are new to this concept.