docs: add installation section

closes #233

Author: ryhinchey

README.md

@@ -9,6 +9,16 @@ HTTP request logger middleware for node.js
 
 > Named after [Dexter](http://en.wikipedia.org/wiki/Dexter_Morgan), a show you should not watch until completion.
 
+## Installation
+
+This is a [Node.js](https://nodejs.org/en/) module available through the
+[npm registry](https://www.npmjs.com/). Installation is done using the
+[`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):
+
+```sh
+$ npm install morgan
+```
+
 ## API
 
 <!-- eslint-disable no-unused-vars -->