Add dedicated package README

[martincostello]

• Add a dedicated package README to improve rendering in the NuGet gallery.
• Fix markdownlint warnings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.78%. Comparing base (82e80d0) to head (c474aeb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #665   +/-   ##
=======================================
  Coverage   97.78%   97.78%           
=======================================
  Files          15       15           
  Lines         271      271           
  Branches       37       37           
=======================================
  Hits          265      265           
  Misses          3        3           
  Partials        3        3           

Flag	Coverage Δ
linux	97.04% <ø> (ø)
macos	97.78% <ø> (ø)
windows	97.04% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.