Add shields

Author: ErikSchierboom

README.md

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 # Exercism .NET Tests Package
 
+![NuGet Version](https://img.shields.io/nuget/v/exercism.tests?label=exercism.tests) ![NuGet Version](https://img.shields.io/nuget/v/exercism.tests.xunit.v3?label=exercism.tests.xunit.v3)
+
 This package adds some additional functionality to be used in the [C#](https://exercism.org/tracks/csharp/), [F#](https://exercism.org/tracks/fsharp/) and [VB](https://exercism.org/tracks/vbnet/) tracks on Exercism.