Added Nuget badge

Author: adriangodong

README.md

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 This repository contains the open source SDK for integrating [Medium's OAuth2 API](https://github.com/Medium/medium-api-docs) into your .NET app.
 
 [![Build status](https://ci.appveyor.com/api/projects/status/u1lmowf82gdckxmo?svg=true)](https://ci.appveyor.com/project/adriangodong/medium-sdk-dotnet)
+[![NuGet](https://img.shields.io/nuget/v/SimpleObjectFactory.svg)](https://www.nuget.org/packages/SimpleObjectFactory/)
 
 Install
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