add nuget version badges.

README.zh-cn.md

@@ -1,6 +1,7 @@
 # CAP
 [![Travis branch](https://img.shields.io/travis/dotnetcore/CAP/master.svg?label=travis-ci)](https://travis-ci.org/dotnetcore/CAP)
 [![AppVeyor](https://ci.appveyor.com/api/projects/status/4mpe0tbu7n126vyw?svg=true)](https://ci.appveyor.com/project/yuleyule66/cap)
+[![NuGet](https://img.shields.io/nuget/vpre/DotNetCore.CAP.svg)](https://www.nuget.org/packages/DotNetCore.CAP/)
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/dotnetcore/CAP/master/LICENSE.txt)
 
 CAP 是一个在分布式系统（SOA、MicroService）中实现最终一致性的库，它具有轻量级、易使用、高性能等特点。