Update readme instructions [skip ci]

Modifying the 'add reference' instructions to use NuGet to clarify how to specifically do that

Author: timheuer

README.md

@@ -99,7 +99,11 @@ To use this, do the following:
    cd DemoApi 
    ```
 
-1. Add a reference to `Fritz.InstantAPIs.dll`
+1. Add the NuGet package for [Fritz.InstantAPIs](https://www.nuget.org/packages/Fritz.InstantAPIs/):
+
+   ```bash
+   dotnet add package Fritz.InstantAPIs --prerelease
+   ```
 
 1. In *Program.cs*, add the following namespace: