docs(README): explain what the loader does (#156)

Author: simon04

README.md

@@ -11,6 +11,7 @@
       src="https://webpack.js.org/assets/icon-square-big.svg">
   </a>
   <h1>File Loader</h1>
+  <p>Instructs webpack to emit the required object as file and to return its public url.</p>
 </div>
 
 <h2 align="center">Install</h2>
@@ -152,4 +153,4 @@ MIT
 [cover-url]: https://coveralls.io/github/webpack-contrib/file-loader
 
 [chat]: https://badges.gitter.im/webpack/webpack.svg
-[chat-url]: https://gitter.im/webpack/webpack
+[chat-url]: https://gitter.im/webpack/webpack