Improve README suggestion for async indexing with after_save_commit

* after_commit solves testing problems by ensuring create is committed
* Use rails 6's after_save_commit to avoid reindexing on destroy

Author: trappist

elasticsearch-model/README.md

@@ -506,8 +506,8 @@ with a tool like [_Resque_](https://github.com/resque/resque) or [_Sidekiq_](htt
 class Article
   include Elasticsearch::Model
 
-  after_save    { Indexer.perform_async(:index,  self.id) }
-  after_destroy { Indexer.perform_async(:delete, self.id) }
+  after_save_commit { Indexer.perform_async(:index,  self.id) }
+  after_destroy     { Indexer.perform_async(:delete, self.id) }
 end
 ```