Update readme with CI NET Core Badge

Update readme with CI NET Core Badge from Github Actions

Author: fernandezja

README.md

@@ -2,6 +2,7 @@
 Generate color based on the given string. C# port of [ColorHash Javascript Library](https://github.com/zenozeng/color-hash).
 
 [![NuGet version (ColorHashSharp)](https://img.shields.io/nuget/v/ColorHashSharp.svg?style=flat-square)](https://www.nuget.org/packages/ColorHashSharp/)
+![.NET Core](https://github.com/fernandezja/ColorHashSharp/workflows/.NET%20Core/badge.svg?branch=master)
 [![Build status](https://fernandezja.visualstudio.com/ColorHashSharp/_apis/build/status/ColorHashSharp-CI)](https://fernandezja.visualstudio.com/ColorHashSharp/_build/latest?definitionId=4)
 
 #### Basic
@@ -29,4 +30,4 @@ var hex = colorHash.Hex("Hello World");
 
 Debug.WriteLine($"  hex = {hex}");
 //  hex = 8796C5
-```
+```