add readme

Signed-off-by: Haidong <whaidong@amazon.com>

data-prepper-plugins/mutate-event-processors/README.md

@@ -261,8 +261,11 @@ and the type conversion processor will change it to the following output, where
 {"message": "10.10.10.10 [19/Feb/2015:15:50:36 -0500] 200", "clientip":"10.10.10.10", "timestamp": "19/Feb/2015:15:50:36 -0500", "response_status": 200}
 ```
 ### Configuration
-* `key` - (required) - keys whose value needs to be converted to a different type
+* `key` - keys whose value needs to be converted to a different type. Required if `keys` option is not defined.
+* `keys` - list of keys whose value needs to be converted to a different type. Required if `key` option is not defined. 
 * `type` - target type for the value of the key. Possible values are `integer`, `double`, `string`, and `boolean`. Default is `integer`.
+* `null_values` - treat any value in the null_values list as null.
+  * Example: `null_values` is `["-"]` and `key` is `key1`. `{"key1": "-", "key2": "value2"}` will parse into `{"key2": "value2"}`
 
 ## List-to-map Processor
 A processor that converts a list of objects from an event, where each object has a key field, to a map of keys to objects.