docs: add preamble to install section

Author: dougwilson

README.md

@@ -8,6 +8,10 @@
 
 ## 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):
+
 ```bash
 $ npm install express-session
 ```