Add note about when to import bootstrap CSS. (facebook#1618)

* Add note about when to import bootstrap CSS.

* Tweak

packages/react-scripts/template/README.md

@@ -560,11 +560,13 @@ npm install react-bootstrap --save
 npm install bootstrap@3 --save
 ```
 
-Import Bootstrap CSS and optionally Bootstrap theme CSS in the ```src/index.js``` file:
+Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your ```src/index.js``` file:
 
 ```js
 import 'bootstrap/dist/css/bootstrap.css';
 import 'bootstrap/dist/css/bootstrap-theme.css';
+// Put any other imports below so that CSS from your
+// components takes precedence over default styles.
 ```
 
 Import required React Bootstrap components within ```src/App.js``` file or your custom component files: