docs: add contributing guidelines

CONTRIBUTING.md

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+# Contributing to the PAYONE Commerce Platform Node SDK
+
+Thank you for considering contributing to the PAYONE Commerce Platform Node SDK! We appreciate your efforts to help improve this project. Below are guidelines for contributing.
+
+## How to Contribute
+
+### Pull Requests
+
+We welcome pull requests! Please follow these steps to submit one:
+
+1. **Fork** the repository and create your branch from `main`.
+2. **Install** any dependencies and ensure the project builds and passes tests.
+
+   ```bash
+   npm install
+   ```
+
+3. **Develop** your changes in Typescript.
+4. **Test** your changes thoroughly.
+   ```bash
+   npm run test
+   ```
+5. **Write** clear, concise, and self-explanatory commit messages.
+6. **Open** a pull request with a clear title and description of what your change does.
+7. **Ensure** your pull request follows the [style guides](#style-guides).
+
+### Reporting Bugs
+
+If you encounter any bugs, please report them using one of the following methods:
+
+1. **Issue Tracker**: Submit an issue through our [issues tracker](https://github.com/PAYONE-GmbH/PCP-server-nodeJS-SDK/issues/new).
+2. **Security Issues**: For security-related issues, please contact our IT support via email at [email protected] with a clear subject line indicating that it is a security issue. This ensures that the issue will be visible to and handled by the PAYONE tech support team.
+
+## Style Guides
+
+### Git Commit Messages
+
+We use [Conventional Commits](https://www.conventionalcommits.org/) for our commit messages. See the whole specification [here](https://www.conventionalcommits.org/en/v1.0.0/#specification).
+
+### TypeScript
+
+- Follow the [TypeScript Handbook](https://www.typescriptlang.org/docs/handbook/intro.html) for best practices.
+- Ensure type definitions are clear and comprehensive.
+- We are using `strict` mode in `tsconfig.json` to enforce type safety:
+  ```json
+  {
+    "compilerOptions": {
+      "strict": true,
+      ...
+    }
+  }
+  ```
+
+### Code Style
+
+- Follow the project's existing code style.
+- Use tools like [ESLint](https://eslint.org/) and [Prettier](https://prettier.io/) to maintain code quality and consistency.
+- Ensure your editor/IDE is configured to follow the project's coding standards.
+
+### Testing
+
+- Write unit tests using [Vitest](https://vitest.dev/).
+- Ensure new features and bug fixes are covered by tests.
+- Run the test suite to confirm all tests pass before submitting your pull request.
+
+Thank you for your contributions!