Adds a README file to the NuGet package

Includes the README.md file in the generated NuGet package.
This allows users browsing the package to view the project's
README directly from the package details page.

Merge pull request #121 from exceptionless/modernize/net8-net10

Modernize library: target net8.0/net10.0, modern C# idioms, xUnit v3

Remove github secret

Fix build

Update to latest build / CI process

Sign test project