Fixes link, add openapi feature file dummy

[spacether]

• fixes readme spec link
• adds placeholder file to store openapi features, which is an experimental feature for https://github.com/openapi-json-schema-tools/openapi-implementation-tracker

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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