#36 Update to target multiple frameworks (netcoreapp3.1 and net5.0)

[mikkou-adafy]

Resolves #36

• Update library projects and tests to target both .NET Core 3.1 and .NET 5.0

[mikoskinen]

Looking good. Could you add couple things:

1. ASP.NET Core 5.0 sample. Functionality could be identical to the current WebApp-sample.
2. Readme updated with a link to the new sample.

Thanks in advance :)

[mikkou-adafy]

Added a .NET 5.0 Web API sample project and updated the README.