docs: Add ci status badge for main branch

Author: AdrianDBS

README.md

@@ -1,5 +1,6 @@
 # Minuit2.NET
 
+[![Main Branch CI Status](https://img.shields.io/github/actions/workflow/status/bruker-biosensors/minuit2.net/minuit2-net-pipeline.yml?branch=main&label=main)](https://github.com/bruker-biosensors/minuit2.net/actions/workflows/minuit2-net-pipeline.yml)
 [![NuGet Version](https://img.shields.io/nuget/v/minuit2.net.svg)](https://www.nuget.org/packages/minuit2.net/)
 
 A .NET wrapper for the [Minuit2](https://root.cern.ch/doc/master/Minuit2Page.html) library from CERN's [ROOT project](https://root.cern/), enhanced with practical extensions.