Add README.md to NuGet package

NuGet.org now shows README.md file if you include it into the package.

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

Problem is our README is in .rst format. The options are:

  1. new README.md tailored to .NET
  2. rewrite our .rst into .md

I prefer option 1 because our README has info for both embedding into Python and into .NET, but NuGet package is tailored only for embedding into .NET.