Update README.md

Author: danbovey

README.md

@@ -56,6 +56,34 @@ import InfiniteScroll from 'react-infinite-scroller';
 </div>
 ```
 
+### Custom parent element
+
+You can define a custom `parentNode` element to base the scroll calulations on.
+
+```
+class InfiniteScrollOverride extends InfiniteScroll {
+
+    /**
+     * We are overriding the getParentElement function to use a custom element as the scrollable element
+     *
+     * @param {any} el the scroller domNode
+     * @returns {any} the parentNode to base the scroll calulations on
+     *
+     * @memberOf InfiniteScrollOverride
+     */
+    getParentElement(el) {
+        if (this.props.scrollParent) {
+            return this.props.scrollParent;
+        }
+        return super.getParentElement(el);
+    }
+
+    render() {
+        return super.render();
+    }
+}
+```
+
 ## Props
 
 | Name             | Type          | Default    | Description|