Include Readme in NuGet package

[nickfloyd]

Sourced from: https://github.com/octokit/webhooks.net/issues/107 (hat tip @JamieMagee)

https://docs.microsoft.com/en-gb/nuget/nuget-org/package-readme-on-nuget-org https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#packagereadmefile