chore(docs): Create example for injectedJavaScript (#571)

docs/Reference.md

@@ -115,6 +115,22 @@ Set this to provide JavaScript that will be injected into the web page when the
 
 To learn more, read the [Communicating between JS and Native](Guide.md#communicating-between-js-and-native) guide.
 
+Example:
+
+Post message a JSON object of `window.location` to be handled by [`onMessage`](Reference.md#onmessage)
+
+```jsx
+const INJECTED_JAVASCRIPT = `(function() {
+    window.ReactNativeWebView.postMessage(JSON.stringify(window.location));
+})();`;
+
+<WebView
+  source={{ uri: 'https://facebook.github.io/react-native' }}
+  injectedJavaScript={INJECTED_JAVASCRIPT}
+  onMessage={this.onMessage}
+/>
+```
+
 ---
 
 ### `mediaPlaybackRequiresUserAction`