Add package README

[martincostello]

Add missing package readme for Swashbuckle.AspNetCore to avoid warning on publish.

Run dotnet nuget push "*.nupkg" --api-key *** --skip-duplicate --source https://api.nuget.org/v3/index.json
Pushing Swashbuckle.AspNetCore.7.1.0.nupkg to 'https://www.nuget.org/api/v2/package'...
  PUT https://www.nuget.org/api/v2/package/
warn : Readme missing. Go to https://aka.ms/nuget-include-readme learn How to include a readme file within the package.
  Created https://www.nuget.org/api/v2/package/ 258ms
Your package was pushed.

The solution used for all the other packages doesn't work for this meta-package as it uses a .nuspec file so the MSBuild properties don't apply.