docs(user-explorer): add fields

Author: Matteo Gabriele

docs/user-explorer.md

@@ -5,9 +5,10 @@ Add the `userId` on first load just passing it in the options object
 ```js
 Vue.use(VueAnalytics, {
   id: 'UA-XXX-X',
-  userId: 'xxx'
+  fields: {
+    userId: 'xxx'
+  }
 })
 ```
 
 **it is also possible to set the **`userId`** in runtime using the **[**set**](/set.md)** method**
-