Update the IntersectionObserver polyfill info

docs/plugins/impression-tracker.md

@@ -20,14 +20,33 @@ ga('require', 'impressionTracker', options);
 
 The `impressionTracker` plugin takes advantage of a new browser API called [`IntersectionObserver`](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API), which allows you to register a callback that gets invoked whenever a target element is visible within the viewport.
 
-The `IntersectionObserver` is supported natively in Chrome 51+, and with an [`IntersectionObserver` polyfill](https://cdn.rawgit.com/philipwalton/IntersectionObserver/61cd29/polyfill/intersection-observer.js), it can be used in all other browsers as well.
+The `IntersectionObserver` API is supported natively in Chrome 51+, and with an [`IntersectionObserver` polyfill](https://github.com/WICG/IntersectionObserver/tree/gh-pages/polyfill), it can be used in all other browsers as well.
 
 To use the polyfill, add it to your page prior to requiring the `impressionTracker` plugin:
 
+The `IntersectionObsever` polyfill is available on npm, and can be installed by running the following command:
+
+```
+npm install intersection-observer
+```
+
+Then link to the `intersection-observer.js` script file in your HTML page:
+
+
 ```html
-<script src="https://cdn.rawgit.com/philipwalton/IntersectionObserver/61cd29/polyfill/intersection-observer.js"></script>
+<script src="path/to/intersection-observer.js"></script>
 ```
 
+The `IntersectionObsever` polyfill is also available from [polyfill.io](https://polyfill.io). An advantage of using `polyfill.io` is the service automatically detects browser support and only delivers the polyfill (and any needed dependencies) if the requesting brower lacks native support.
+
+You can link to the CDN version on `polyfill.io` with the following script:
+
+```html
+<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>
+```
+
+See the [`polyfill.io` documentation](https://polyfill.io/v2/docs/) for more information on using the service.
+
 ## Options
 
 The following table outlines all possible configuration options for the `impressionTracker` plugin. If any of the options has a default value, the default is explicitly stated: