Add README to NuGet packages

[kurtschelfthout]

Low priority.

Nuget.org is highlighting that they expect a README for the package. Seems relatively easy:

https://devblogs.microsoft.com/nuget/add-a-readme-to-your-nuget-package/#add-a-readme-to-your-package

There is already a description, so I don't think the top-level README.md in this repo is appropriate to include.

For the main FsCheck package, I'd instead extract https://fscheck.github.io/FsCheck//QuickStart.html#A-Simple-Example to a file, check it in and include that with the nuget package. It's ok if that is a manual step, though I guess bonus points for doing it automatically somehow (that would mean we don't have to maintain the same doc multiple times!)

For the others perhaps something similar (a small usage example) can be put together/extracted from the existing docs.

• Add Nuget readme for FsCheck
• Add Nuget readme for FsCheck.Xunit
• Add Nuget readme for FsCheck.NUnit