adding configuration documentation

Author: agounaris

DependencyInjection/Configuration.php

@@ -71,6 +71,15 @@
  *   - [level]: level name or int value, defaults to DEBUG
  *   - [bubble]: bool, defaults to true
  *
+ * - elasticsearch:
+ *   - elasticsearch:
+ *      - host: elastic search host name
+ *      - [port]: defaults to 9200
+ *      - [index]: index name, defaults to monolog
+ *      - [index_type]: index_type name, defaults to logs
+ *   - [level]: level name or int value, defaults to DEBUG
+ *   - [bubble]: bool, defaults to true
+ *
  * - fingers_crossed:
  *   - handler: the wrapped handler's name
  *   - [action_level|activation_strategy]: minimum level or service id to activate the handler, defaults to WARNING