Update README.md

Author: adoconnection

README.md

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 NETCore 3.1.8 Razor Template Engine. No legacy code.
 * .NET Standard 2.0
 * .NET Framework 4.7.2
+* Windows / Linux
 
 [![NuGet](https://img.shields.io/nuget/dt/RazorEngineCore.svg?style=flat-square)](https://www.nuget.org/packages/RazorEngineCore)
 [![NuGet](https://img.shields.io/nuget/v/RazorEngineCore.svg?style=flat-square)](https://www.nuget.org/packages/RazorEngineCore)