add doc link && test=serving

doc/MODEL_ENSEMBLE_IN_PADDLE_SERVING.md

 # Model Ensemble in Paddle Serving
 
+([简体中文](MODEL_ENSEMBLE_IN_PADDLE_SERVING_CN.md)|English)
+
 In some scenarios, multiple models with the same input may be used to predict in parallel and integrate predicted results for better prediction effect. Paddle Serving also supports this feature.
 
 Next, we will take the text classification task as an example to show model ensemble in Paddle Serving (This feature is still serial prediction for the time being. We will support parallel prediction as soon as possible).

doc/MODEL_ENSEMBLE_IN_PADDLE_SERVING_CN.md

 # Paddle Serving中的集成预测
 
+(简体中文|[English](MODEL_ENSEMBLE_IN_PADDLE_SERVING.md))
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 在一些场景中，可能使用多个相同输入的模型并行集成预测以获得更好的预测效果，Paddle Serving提供了这项功能。
 
 下面将以文本分类任务为例，来展示Paddle Serving的集成预测功能（暂时还是串行预测，我们会尽快支持并行化）。