docs(readme): function syntax gets executed once

Closes #117

README.md

@@ -66,12 +66,14 @@ var vm = new Vue({
   el: '#demo',
   firebase: function () {
     return {
-      anArray: db.ref('url/to/my/collection')
+      anArray: db.ref('url/to/my/collection/')
     }
   }
 })
 ```
 
+⚠️: This function will get executed only once. If you want to have automatic rebind (pretty much like a computed property) use a `$watch` and call `$unbind` and then `$bindAsArray`
+
 **About the cancelCallback**:
 
 > An optional callback that will be notified if your event subscription is ever canceled because your client does not have permission to read this data (or it had permission but has now lost it). This callback will be passed an `Error` object indicating why the failure occurred.