Update ABTEST_IN_PADDLE_SERVING.md

doc/ABTEST_IN_PADDLE_SERVING.md

 # ABTEST in Paddle Serving
 
+([简体中文](./ABTEST_IN_PADDLE_SERVING_CN.md)|English)
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 This document will use an example of text classification task based on IMDB dataset to show how to build a A/B Test framework using Paddle Serving. The structure relationship between the client and servers in the example is shown in the figure below.
 
 <img src="abtest.png" style="zoom:33%;" />