Add package badge in readme (#86)

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Author: zhiyuanliang-ms

README.md

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 # Azure App Configuration - JavaScript Provider
 
+[![app-configuration-provider](https://img.shields.io/npm/v/@azure/app-configuration-provider?label=@azure/app-configuration-provider)](https://www.npmjs.com/package/@azure/app-configuration-provider)
+
 The [Azure App Configuration](https://docs.microsoft.com/en-us/azure/azure-app-configuration/overview) provider for JavaScript enables developers to configure their applications using centralized configuration located in Azure App Configuration. 
 
 ## Getting started