fix(api): Fix default class export in API

Author: wcandillon

README.md

@@ -12,12 +12,14 @@ Progressively load images using a blur effect.
 $ npm install react-progressive-image-loading --save
 ```
 
-## Usage
+## Import
 
 ```jsx
 import ProgressiveImage from "react-progressive-image-loading";
 ```
 
+## Usage
+
 ```jsx
 <ProgressiveImage
     preview="/images/tiny-preview.png"