Update readme to include link to management SDK

Author: yzhong94

readme.md

@@ -9,6 +9,7 @@ This repository contains the following:
 The API reference documentation for .NET SDK is [here][dotnet-api-reference].
 
 To find SDKs in other languages for Azure IoT, please refer to the [azure-iot-sdks][azure-iot-sdks] repository.
+For IoT Hub Management SDK in .NET, please visit [azure-sdk-for-net](https://github.com/Azure/azure-sdk-for-net) repository
 
 ## Need Support?
 * Have a feature request for SDKs? Please post it on [User Voice](https://feedback.azure.com/forums/321918-azure-iot) to help us prioritize.