add note about version (#50058)

Author: gewarren

docs/core/testing/unit-testing-fsharp-with-xunit.md

@@ -62,6 +62,9 @@ The test project requires other packages to create and run unit tests. `dotnet n
 dotnet reference add ../MathService/MathService.fsproj
 ```
 
+> [!TIP]
+> If you're using .NET 9 SDK or earlier, use the "verb first" form (`dotnet add reference`) instead. The "noun first" form was introduced in .NET 10.
+
 You can see the entire file in the [samples repository](https://github.com/dotnet/samples/blob/main/core/getting-started/unit-testing-with-fsharp/MathService.Tests/MathService.Tests.fsproj) on GitHub.
 
 You have the following final solution layout: