Update README.md

Author: bitfaster

README.md

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 High performance, thread-safe in-memory caching primitives for .NET.
 
-[![NuGet version](https://badge.fury.io/nu/BitFaster.Caching.svg)](https://badge.fury.io/nu/BitFaster.Caching)
+[![NuGet version](https://badge.fury.io/nu/BitFaster.Caching.svg)](https://badge.fury.io/nu/BitFaster.Caching) ![.NET Core](https://github.com/bitfaster/BitFaster.Caching/workflows/.NET%20Core/badge.svg)
 
 # Installing via NuGet
 `Install-Package BitFaster.Caching`