Add polyfill section to readme

Author: thebuilder

README.md

@@ -33,6 +33,21 @@ or NPM:
 npm install react-intersection-observer --save
 ```
 
+### Polyfill for intersection-observer
+The component requires the [intersection-observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) to be available on the global namespace. At the moment it is included in Chrome, Firefox, Opera and Edge. Make sure to include the polyfill to support other browsers like IE and Webkit.
+
+You can import the [polyfill](https://github.com/jeremenichelli/intersection-observer-polyfill) directly or use a service like [polyfill.io](https://polyfill.io/v2/docs/).
+
+```sh
+yarn add intersection-observer-polyfill
+```
+
+Then import it in your app
+
+```js
+import 'intersection-observer-polyfill'
+```
+
 ## Props
 The **`<Observer />`** accepts the following props: