fix according to comments

serve/README.md

@@ -6,9 +6,14 @@ PaddlePaddle. It provides an HTTP endpoint.
 ## Run
 
 The inference server reads a trained model (a topology file and a
-parameter file) and serves HTTP request at port `8000`. **The HTTP API
-will differ slightly for each model,** please see [HTTP
-API](#http-api) for details.
+parameter file) and serves HTTP request at port `8000`. Because models
+differ in the numbers and types of inputs, **the HTTP API will differ
+slightly for each model,** please see [HTTP API](#http-api) for the
+API spec,
+and
+[here](https://github.com/PaddlePaddle/book/wiki/PaddlePaddle-Book-pretrained-model) for
+the request examples of different models that illustrate the
+difference.
 
 We will first show how to obtain the PaddlePaddle model, and then how
 to start the server.