Add note about $router in navigation (#1304)

* Add note about $router in navigation

Closes #1136

* Update navigation.md

Author: posva

docs/en/essentials/navigation.md

@@ -4,6 +4,8 @@ Aside from using `<router-link>` to create anchor tags for declarative navigatio
 
 #### `router.push(location, onComplete?, onAbort?)`
 
+**Note: Inside of a Vue instance, you have access to the router instance as `$router`. You can therefore call `this.$router.push`.**
+
 To navigate to a different URL, use `router.push`. This method pushes a new entry into the history stack, so when the user clicks the browser back button they will be taken to the previous URL.
 
 This is the method called internally when you click a `<router-link>`, so clicking `<router-link :to="...">` is the equivalent of calling `router.push(...)`.