feat(router): mark the index property as deprecated

Use {path: '', component: A} instead of {index: true, component: A}

}#

modules/@angular/router/src/config.ts

@@ -3,6 +3,10 @@ import {Type} from '@angular/core';
 export type RouterConfig = Route[];
 
 export interface Route {
+  /**
+   * Use `path: ''` instead.
+   * @deprecated
+   */
   index?: boolean;
   path?: string;
   terminal?: boolean;