filter_wasm: Add a description for event_format (#1293)

Signed-off-by: Hiroshi Hatake <[email protected]>

Author: cosmo0920

pipeline/filters/wasm.md

@@ -19,6 +19,7 @@ The plugin supports the following configuration parameters:
 | Key | Description |
 | :--- | :--- |
 | Wasm\_Path | Path to the built Wasm program that will be used. This can be a relative path against the main configuration file. |
+| Event\_Format | Define event format to interact with Wasm programs: msgpack or json. Default: json |
 | Function\_Name | Wasm function name that will be triggered to do filtering. It's assumed that the function is built inside the Wasm program specified above. |
 | Accessible\_Paths | Specify the whilelist of paths to be able to access paths from WASM programs. |
 
@@ -34,6 +35,7 @@ Here is a configuration example.
 [FILTER]
     Name wasm
     Match dummy.*
+    Event_Format json # or msgpack
     WASM_Path /path/to/wasm_program.wasm
     Function_Name filter_function_name
     Accessible_Paths .,/path/to/accessible