Mistake in adding CSS Preprocessor documentation

Created by: KevinBacas

Description

Hello everyone, I am reporting an error in the documentation concerning this section.

In fact, if you follow this tutorial, you modify the "build" script to "build": "react-scripts build && npm run build-css". There is two problems here:

You run react-scripts build before processing sass files. So you may not have the latest version of the css file.
On a CI server, if you start only from the sources from git, you won't have any .css file created so it returns an error (see below).

Expected behavior

The build script should be: npm run build-css && react-scripts build. So it prevents those issues.

Actual behavior

$ react-scripts build && npm run build-css
Creating an optimized production build...
Failed to compile.

Module not found: Error: Cannot resolve 'file' or 'directory' ./index.css

Environment

Run these commands in the project folder and fill in their results:

npm ls react-scripts (if you haven’t ejected):

my-app@0.1.0 C:\Users\Kevbac\Documents\git\my-app
`-- react-scripts@0.9.0

node -v: v6.9.0
npm -v: 3.10.8

Then, specify:

Operating system: Windows 10
Browser and version:

Reproducible Demo

I just followed the turtorial with a CRA app from scratch.

git clone https://github.com/KevinBacas/my-app
cd my-app
npm install

npm run build # This will throw an error

BTW thank you for documenting this, I'll submit a PR with the little fix attached to this issue.