Update README

Author: johnsusek

README.md

@@ -111,7 +111,10 @@ You can use the following config options:
   "dataPath": { // The path to a folder that the server can use to store data and temporary files.
     "relative": true, // Whether to use a path relative to the `elastalertPath` folder.
     "path": "/server_data" // The path to the data folder.
-  }
+  },
+  "es_host": "elastalert", // For getting metadata and field mappings, connect to this ES server
+  "es_port": 9200, // Port for above
+  "writeback_index": "elastalert_status" // Writeback index to examine for /metadata endpoint
 }
 ```