feat: add constructor hint

Author: Hazlank

src/index.js

@@ -39,6 +39,10 @@ export default class VueRouter {
   afterHooks: Array<?AfterNavigationHook>
 
   constructor (options: RouterOptions = {}) {
+    if (!(this instanceof VueRouter)) {
+      warn('router must be called with the new operator.')
+    }
+
     this.app = null
     this.apps = []
     this.options = options