Update readme.md

Author: David Horak

readme.md

@@ -50,6 +50,7 @@ To see the details code documentation, please read the [Code Documentation](http
  - [go](#go)
  - [routeURL](#routeurl)
  - [navigationGuard](#navigationguard)
+ - [onBeforeNavigation](#onbeforenavigation)
  - [onNavigationChanged](#onnavigationchanged)
  - [onError](#onerror)
  - [Location Object](#location-object)
@@ -623,6 +624,24 @@ Parameters:
 
 to, from are [Route Object](#route-object).
 
+#### onBeforeNavigation
+Register a callback which will be called before execution of navigation guards.
+```javascript
+const unregister = $router.onBeforeNavigation(callback);
+
+// To unregister the event:
+unregister();
+unregister = null;
+
+```
+Parameters:
+
+| Name | Description | Type |
+| :----------- | :---------------------------------------- | :------------------------------------ |
+| callback | callback function with fn(to, from) signature. | function |
+
+to, from are [Route Object](#route-object).
+
 #### onNavigationChanged
 Register a callback which will be called when all navigation guards are resolved, and the final navigation change is resolved.
 ```javascript