README: Add note about persistent HTTP connections

Also update the example and add esutil.BulkIndexer to the list of helpers.

(cherry picked from commit bbd6b17)

Author: karmi

README.md

@@ -98,6 +98,7 @@ if err != nil {
   log.Fatalf("Error getting response: %s", err)
 }
 
+defer res.Body.Close()
 log.Println(res)
 
 // [200 OK] {
@@ -106,6 +107,8 @@ log.Println(res)
 // ...
 ```
 
+> NOTE: It is _critical_ to both close the response body _and_ to consume it, in order to re-use persistent TCP connections in the default HTTP transport. If you're not interested in the response body, call `io.Copy(ioutil.Discard, res.Body)`.
+
 When you export the `ELASTICSEARCH_URL` environment variable,
 it will be used to set the cluster endpoint(s). Separate multiple adresses by a comma.
 
@@ -353,8 +356,7 @@ The `estransport` package handles the transfer of data to and from Elasticsearch
 
 ## Helpers
 
-The `esutil` package provides convenience helpers for working with the client. At the moment, it provides the
-`esutil.JSONReader()` helper function.
+The `esutil` package provides convenience helpers for working with the client. At the moment, it provides the `esutil.JSONReader()` and the `esutil.BulkIndexer` helpers.
 
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