Improve documentation - Githubissues

Both public NuGet packages share the same README.md file from the repository root.

[x] Create copies of the current README.md files for every NuGet package and adjust project files accordingly
[x] Make every README.md file unique and describe only features/details specific to this package
[x] Make the root README.md file more generic and provide information about both packages
[x] Update XML documentation for the DateTimeOnly.Json public classes

OlegRa / System.DateTimeOnly