doc: add npm link to README

PR-URL: https://github.com/nodejs/node/pull/7894
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

README.md

@@ -4,8 +4,8 @@
 
 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
+efficient. The Node.js package ecosystem, [npm][], is the largest ecosystem of
-source libraries in the world.
+open source libraries in the world.
 
 The Node.js project is supported by the
 [Node.js Foundation](https://nodejs.org/en/foundation/). Contributions,
@@ -389,6 +389,7 @@ keys:
 * **Timothy J Fontaine** <tjfontaine@gmail.com>
 `7937DFD2AB06298B2293C3187D33FF9D0246406D`
 
+[npm]: https://www.npmjs.com
 [Website]: https://nodejs.org/en/
 [Contributing to the project]: CONTRIBUTING.md
 [Node.js Help]: https://github.com/nodejs/help