adding documentation to the readme for this new functionality described in issue ncb000gt#23

Author: brozeph

README.md

@@ -47,6 +47,14 @@ var
   es = elasticsearch.createClient(config);
 ```
 
+You may also supply a logger for each request by passing in an optional second argument to `elasticsearch.createClient(config, requestLogger)`. This will result in every http(s) request being logged via the callback supplied to the `requestLogger` parameter.
+
+```Javascript
+var
+  elasticsearch = require('elasticsearch'),
+  es = elasticsearch.createClient(config, console.log);
+```
+
 ##### config._index
 
 When initializing the library, you may choose to specify an index and/or type to work with at the start to save from having to supply this information in the options for each operation request: