Merge pull request #2 from j-alexander/improve_developer_experience

Improve the developer experience with this version of the library, including:
- documentation improvements (README.md(s), Appveyor.yml, etc)
- debugger symbol inclusion and source links in the NuGet package
- expanded target versions (now net481, netstandard2.0, netcoreapp3.1, net80, and net90), and
- open source license clarity

Merge pull request #1 from j-alexander/update_project

update project files with net80, F# 9, and FSharp.Data 6.6

increase minimum FSharp.Data and FSharp.Core versions