Add router.onReady to Final State Injection (#251)

Add router.onReady to client-side app amount to avoid bailing hydration and performing full client-side render.
e.g. it will cause rehydration fail when the page component is async

Author: guAnsunyata

docs/guide/data.md

@@ -189,7 +189,9 @@ if (window.__INITIAL_STATE__) {
   store.replaceState(window.__INITIAL_STATE__)
 }
 
-app.$mount('#app')
+router.onReady(() => {
+  app.$mount('#app')
+});
 ```
 
 ## Store Code Splitting