Add README for .NET

[mpkorstanje]

🤔 What's the problem you're trying to solve?

When deploying the .Net package we see the warning

Time Elapsed 00:00:06.90
Run dotnet pack --configuration Release --no-build --output NuGetOut
  The package Cucumber.CucumberExpressions.18.0.1 is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
  Successfully created package '/home/runner/work/cucumber-expressions/cucumber-expressions/dotnet/NuGetOut/Cucumber.CucumberExpressions.18.0.1.nupkg'.
Run dotnet nuget push NuGetOut/*.nupkg --api-key ${NUGET_API_KEY} --source https://api.nuget.org/v3/index.json
Pushing Cucumber.CucumberExpressions.18.0.1.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/ 680ms

✨ What's your proposed solution?

Either:

• Add a README.md to the project. It can point to the main read me. For example see Python: https://github.com/cucumber/cucumber-expressions/blob/main/python/README.md
• Or copy the main read me:

  cucumber-expressions/javascript/package.json

  Line 24 in 9459ff3

  "build": "npm run build:cjs && npm run build:esm && cp ../README.md dist",

⛏ Have you considered any alternatives or workarounds?

No response

📚 Any additional context?

No response