Add OpenAPI spec, changelog and readme

.gitignore

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+.idea/

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+The major and minor version numbers reflect changes to the DeepL API 
+(backward-incompatible and backward-compatible, respectively). The patch version
+number is used only for corrections to the OpenAPI specification, for example:
+typos, schema fixes, or adding examples.
+
+
+## [2.0.0]
+Initial release of the OpenAPI specification.
+
+
+[2.0.0]: https://github.com/DeepLcom/openapi/releases/tag/v2.0.0

README.md

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-# openapi
+# DeepL OpenAPI Specification
 
+This project contains an [OpenAPI specification][openapi-specification] of the
+[DeepL API][deepl-api], in [YAML](openapi.yaml) and [JSON](openapi.json)
+formats.
+
+You can use this specification to explore the API in tools like
+[Postman][postman], or to auto-generate documentation, SDKs, and code libraries
+using tools such as [Swagger Editor][swagger-editor] or
+[OpenAPI Generator][openapi-generator].
+
+[Changelog](CHANGELOG.md)
+
+If you encounter issues while using this OpenAPI specification or have feature
+requests, please [create an issue][issues].
+
+[deepl-api]: https://www.deepl.com/pro-api
+
+[issues]: https://github.com/DeepLcom/openapi/issues
+
+[openapi-specification]: https://openapis.org/
+
+[openapi-generator]: https://openapi-generator.tech/
+
+[postman]: https://www.postman.com/
+
+[swagger-editor]: https://editor.swagger.io/?url=https://raw.githubusercontent.com/DeepLcom/openapi/main/openapi.yaml