Add readme.md to packages

[buehler]

Describe the missing piece of documentation NuGet.org supports readme files now. This should be used to give a small readme for all packages.

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