Nuspec - specify developmentDependency

[dazinator]

Setting the developmentDependency flag in the Nuspec might be a good idea, as it means that if people add GitReleaseNotes as a NuGet package to their project, when they then come to doing a nuget pack on their project, NuGet will recognise GitReleaseNotes as a dev time dependency, rather than a runtime dependency, so it it won't be added / listed as a depedency for their published NuGet package.

https://docs.nuget.org/create/nuspec-reference