docs: update CONTRIBUTORS.md [skip ci]

Author: allcontributors[bot]

CONTRIBUTORS.md

@@ -20,6 +20,7 @@ We use [All Contributors](https://allcontributors.org/) because we recognize tha
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/jdamerow"><img src="https://avatars.githubusercontent.com/u/8881141?v=4?s=100" width="100px;" alt="Julia Damerow"/><br /><sub><b>Julia Damerow</b></sub></a><br /><a href="https://github.com/dh-tech/undate-python/commits?author=jdamerow" title="Code">💻</a> <a href="https://github.com/dh-tech/undate-python/pulls?q=is%3Apr+reviewed-by%3Ajdamerow" title="Reviewed Pull Requests">👀</a> <a href="https://github.com/dh-tech/undate-python/commits?author=jdamerow" title="Tests">⚠️</a> <a href="#eventOrganizing-jdamerow" title="Event Organizing">📋</a> <a href="#ideas-jdamerow" title="Ideas, Planning, & Feedback">🤔</a></td>
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/maltevogl"><img src="https://avatars.githubusercontent.com/u/20907912?v=4?s=100" width="100px;" alt="Malte Vogl"/><br /><sub><b>Malte Vogl</b></sub></a><br /><a href="https://github.com/dh-tech/undate-python/commits?author=maltevogl" title="Code">💻</a> <a href="https://github.com/dh-tech/undate-python/pulls?q=is%3Apr+reviewed-by%3Amaltevogl" title="Reviewed Pull Requests">👀</a> <a href="https://github.com/dh-tech/undate-python/commits?author=maltevogl" title="Tests">⚠️</a> <a href="https://github.com/dh-tech/undate-python/commits?author=maltevogl" title="Documentation">📖</a></td>
       <td align="center" valign="top" width="14.28%"><a href="http://taylorarnold.org"><img src="https://avatars.githubusercontent.com/u/5752184?v=4?s=100" width="100px;" alt="Taylor Arnold"/><br /><sub><b>Taylor Arnold</b></sub></a><br /><a href="https://github.com/dh-tech/undate-python/pulls?q=is%3Apr+reviewed-by%3Ataylor-arnold" title="Reviewed Pull Requests">👀</a> <a href="#ideas-taylor-arnold" title="Ideas, Planning, & Feedback">🤔</a></td>
+      <td align="center" valign="top" width="14.28%"><a href="https://github.com/rettinghaus"><img src="https://avatars.githubusercontent.com/u/7693447?v=4?s=100" width="100px;" alt="Klaus Rettinghaus"/><br /><sub><b>Klaus Rettinghaus</b></sub></a><br /><a href="#maintenance-rettinghaus" title="Maintenance">🚧</a></td>
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