Updated README.MD (#771)

* Updated README.MD

Include the step to create the index for the model that we'll import.

* Commented to encourage asynchronous index creation

I've added a comment so that users are encouraged to do `create_index!` asynchronously and not at the same time as `import`.

Author: anandvc

README.md

@@ -71,6 +71,10 @@ class Article < ActiveRecord::Base
   include Elasticsearch::Model::Callbacks
 end
 
+# Index creation right at import time is not encouraged.
+# Typically, you would call create_index! asynchronously (e.g. in a cron job)
+# However, we are adding it here so that this usage example can run correctly.
+Article.__elasticsearch__.create_index!
 Article.import
 
 @articles = Article.search('foobar').records