Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to this project! 🎉👍
+
+The following is a set of guidelines for contributing to this repo:
+
+* Use JDK 17 or newer to build the project.
+* Google's Java coding conventions are used for the project. To reformat code, run:
+
+```bash
+./gradlew spotlessApply
+```
+
+* Running some of the tests in classes extending `OpenAIIntegrationTestBase` require to
+  set `OPENAI_API_KEY` environment variable with your
+  API key. Refer to
+  these [instructions](https://platform.openai.com/docs/api-reference/authentication) to create one. No need to run
+  those tests if you don't have an API key or don't want to spend your balance. Can rely on CI executing those instead.
+  😉
+* If your PR modifies a request/response object, please add the changes in `TestDataUtil` and
+  run the `OpenApiSpecificationValidationTest` tests to ensure
+  the [spec](https://github.com/openai/openai-openapi/raw/master/openapi.yaml) is not violated.
+* If your PR adds a new endpoint, please refer to the classes extending `OpenAIClient` for code examples.