doc: add brief Node.js overview to README

Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/4174

README.md

@@ -3,6 +3,11 @@ Node.js
 
 [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
+Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js
+uses an event-driven, non-blocking I/O model that makes it lightweight and
+efficient. The Node.js package ecosystem, npm, is the largest ecosystem of open
+source libraries in the world.
+
 The Node.js project is supported by the
 [Node.js Foundation](https://nodejs.org/en/foundation/). Contributions,
 policies and releases are managed under an