Added instructions on how to install via NPM

Author: ericwbailey

README.md

@@ -14,6 +14,7 @@ There are a few of ways to work with this repo:
 
 - Clone it in its entirety: `https://github.com/ericwbailey/accessible-html-content-patterns.git`
 - [Download a zipped copy](https://github.com/ericwbailey/accessible-html-content-patterns/archive/master.zip)
+- Install via [NPM](https://www.npmjs.com/package/accessible-html-content-patterns): `npm i accessible-html-content-patterns`
 - [Curl](http://explainshell.com/explain/1/curl) just the HTML page (great starting place for a styleguide):
 `curl -o accessible-html-content-patterns.html https://raw.githubusercontent.com/ericwbailey/accessible-html-content-patterns/master/docs/index.html`
 - Copy/paste individual elements from the [source code](https://github.com/ericwbailey/accessible-html-content-patterns/blob/master/docs/index.html)