docs(README): add lead-in description (#517)

Author: skipjack

README.md

@@ -13,6 +13,7 @@
       src="https://webpack.js.org/assets/icon-square-big.svg">
   </a>
   <h1>Extract Text Plugin</h1>
+  <p>Extract text from a bundle, or bundles, into a separate file.</p>
 </div>
 
 <h2 align="center">Install</h2>