update readme for paddle version (#3524)

README.md

@@ -7,6 +7,7 @@ detection models in both industry and research settings. We design
 PaddleDetection to be not only performant, production-ready but also highly
 flexible, catering to research needs.
 
+**Now all models in PaddleDetection require PaddlePaddle version 1.6 or higher, or suitable develop version.**
 
 <div align="center">
   <img src="demo/output/000000570688.jpg" />

README_cn.md

@@ -4,6 +4,8 @@
 
 PaddleDetection的目的是为工业界和学术界提供大量易使用的目标检测模型。PaddleDetection不仅性能完善，易于部署，同时能够灵活的满足算法研发需求。
 
+**目前检测库下模型均要求使用PaddlePaddle 1.6及以上版本或适当的develop版本。**
+
 <div align="center">
   <img src="demo/output/000000570688.jpg" />
 </div>