Document that Corvus.Testing.SpecFlow.NUnit metapackage sets IsTestProject to true

The `Corvus.Testing.SpecFlow.NUnit` includes a reference to the `Microsoft.NET.Test.Sdk` package, which sets the `IsTestProject` MSBuild variable to `true`. This has potentially unexpected side effects.

For example, we recently ran into a problem where a project that build a DLL containing test utilities started using this metapackage. This had the side effect that the build process no longer produced NuGet packages for that DLL. This was not the intention—this project was not a test project, it was a library intended to be used from test projects.

It took a while to diagnose this. It's not entirely obvious that `Microsoft.NET.Test.Sdk` sets `IsTestProject`, nor that this in turn changes the default value for `IsPackable` from `true` to `false`.

When we add documentation for this project, we should clarify this side effect.