[MODEL] Updated the Mongoid example in the README to include `as_inde…

…xed_json` method

Default Mongoid serialization keeps the `_id` property in the JSON as "BSON Object",
which breaks Elasticsearch.

See:

* elastic/elasticsearch#3517
* rails-api/active_model_serializers#354 (comment)

elasticsearch-model/README.md

@@ -535,6 +535,10 @@ class Article
   attr_accessible :id, :title, :published_at
 
   include Elasticsearch::Model
+
+  def as_indexed_json(options={})
+    as_json(except: [:id, :_id])
+  end
 end
 
 Article.create id: '1', title: 'Quick brown fox'