Create CONTRIBUTING.md with contribution guidelines

CONTRIBUTING.md

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+# How to Contribute to PDOK Repositories
+
+Great! Contributions are welcome.
+
+## No Automated Agents or Bot‑Generated Contributions
+
+PDOK repositories do not accept issues, comments, or pull requests created or submitted by AI agents, automated tools, or bots acting without direct human authorship and review.
+All contributions must be:
+
+* reviewed, and submitted by a human
+* accountable and attributable
+* reproducible without dependence on proprietary or undisclosed AI tools
+
+Automated submissions will be closed without review.
+
+## Reporting a Bug
+
+- **Do not open a GitHub issue if the bug is a security vulnerability.**  
+  See the section on [Reporting a Security Issue](#reporting-a-security-issue).
+
+- **Check whether the bug was already reported** by searching the repository’s Issues.
+
+- If no open issue addresses the problem, create a new one using the **bug report template**.  
+  Please include:
+  - a clear and concise title  
+  - a detailed description  
+  - relevant context  
+  - a **code sample** or **executable test case** that demonstrates the problem
+
+## Reporting a Security Issue
+
+- **Do not open a GitHub issue for security vulnerabilities.**
+- Please follow the Kadaster **Responsible Disclosure / Security Policy**:
+  - English: https://www.kadaster.nl/about-us/legal-disclaimer/responsible-disclosure
+  - Dutch: https://www.kadaster.nl/disclaimer/responsible-disclosure
+
+## Contributing a Feature
+
+- Read the project documentation before proposing a feature.  
+  PDOK projects are intentionally opinionated and follow a clearly defined scope. Many codebases are intentionally minimal.
+
+- Create an issue describing the feature you want to add.  
+  We will discuss the proposal with you there.  
+  Not every feature request will be accepted — features must fit the project’s scope.
+
+- Once the feature is agreed upon, **fork the repository** and submit a **pull request** that references the issue.
+
+## Requesting a Feature
+
+- Follow the same process as described in [Contributing a Feature](#contributing-a-feature):  
+  create an issue and describe the feature you would like to see added.
+
+- We read and value all feature requests.  
+  However, we only add features that are beneficial to PDOK and appropriate for the project’s scope.