Update INFERENCE_TO_SERVING.md

doc/INFERENCE_TO_SERVING.md

@@ -2,6 +2,20 @@
 
 ([简体中文](./INFERENCE_TO_SERVING_CN.md)|English)
 
+We should know something before converting to serving model
+
+**inference_model_dir**：the directory of Paddle inference model
+
+**serving_client_dir**: the directory of server side configuration
+
+**serving_client_dir**: the directory of client side configuration
+
+**model_filename**: this is model description file whose default value is `__model__`, if it's not default name, set `model_filename` explicitly
+
+**params_filename**: during `save_inference_model` every Variable will be save as a single file. If we have the inference model whose params are compressed into one file, please set `params_filename` explicitly
+
+
+
 ## Example
 
 ``` python
@@ -12,3 +26,11 @@ serving_server_dir = "serving_server_dir"
 feed_var_names, fetch_var_names = inference_model_to_serving(
 		inference_model_dir, serving_client_dir, serving_server_dir)
 ```
+
+if your model file and params file are both standalone, please use the following api.
+
+```
+feed_var_names, fetch_var_names = inference_model_to_serving(
+		inference_model_dir, serving_client_dir, serving_server_dir,
+		model_filename="model", params_filename="params")
+```