added npm install instructions

Author: ben

README.md

@@ -8,6 +8,12 @@ No curation, no ordering and no tricks, just beautifully justified images.
 
 https://benhowell.github.io/react-grid-gallery/
 
+### Installation
+
+Using [npm](https://www.npmjs.com/):
+
+    $ npm install --save react-grid-gallery
+
 ## Quick (and dirty) Start
 
 ```js