docs: clarify sb init usage (storybookjs#6667)

docs: clarify sb init usage

docs/src/pages/guides/guide-react/index.md

@@ -6,12 +6,14 @@ title: 'Storybook for React'
 ## Automatic setup
 
 You may have tried to use our quick start guide to setup your project for Storybook.
-If it failed because it couldn't detect you're using react, you could try forcing it to use react:
+If it failed because it couldn't detect you're using React, you could try forcing it to use React:
 
 ```sh
 npx -p @storybook/cli sb init --type react
 ```
 
+Note: be sure you have a `package.json` in your project or the above command will fail.
+
 ## Manual setup
 
 If you want to set up Storybook manually for your React project, this is the guide for you.