Update CONTRIBUTORS.md

CONTRIBUTORS.md

-stadtgestalten contributors (sorted alphabeticaly)
-==================================================
+grouprise contributors
+======================
 
 
-* **[Robert](https://stadtgestalten.org/stadt/gestalten/9/conversations/add/)**
-
 
 * **[Konrad Mohrfeldt](https://konradmohrfeldt.farbdev.org/)**
 
   * frontend and other shiny things
 
 * **[Lars](https://stadtgestalten.org/stadt/gestalten/16/conversations/add/)**
+
+* **[Robert](mailto:robert@grouprise.org)**
+
+  * backend and other shiny things
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