kettle 文档

https://missfoxw.github.io/pdi-kettle/#/Kettle%E8%84%9A%E6%9C%AC%E8%BF%90%E8%A1%8C/README?id=_31-%e5%a6%82%e4%bd%95%e5%90%8e%e5%8f%b0%e8%bf%90%e8%a1%8c