README: Adds NuGet badges (#2406)

Replaced useless ADO pipeline badge "never built" with working badges for NuGet packages.

Note: shield.io is a 3rd party website but is considered one of the most popular across OSS

README.md

@@ -1,4 +1,5 @@
-[![Build Status](https://cosmos-db-sdk-public.visualstudio.com/cosmos-db-sdk-public/_apis/build/status/azure-cosmos-dotnet-v3?branchName=master)](https://cosmos-db-sdk-public.visualstudio.com/cosmos-db-sdk-public/_build/latest?definitionId=41&branchName=master)
+[![NuGet](https://img.shields.io/nuget/v/Microsoft.Azure.Cosmos.svg)](https://www.nuget.org/packages/Microsoft.Azure.Cosmos)
+[![NuGet Prerelease](https://img.shields.io/nuget/vpre/Microsoft.Azure.Cosmos.svg)](https://www.nuget.org/packages/Microsoft.Azure.Cosmos)
 
 # Microsoft Azure Cosmos DB .NET SDK Version 3
 
@@ -73,4 +74,3 @@ provided by the bot. You will only need to do this once across all repos using o
 This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/).
 For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or
 contact [opencode@microsoft.com](mailto:opencode@microsoft.com) with any additional questions or comments.
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