update release notes and add sven to maintainer

Author: SvenKlaassen

doc/index.rst

@@ -200,8 +200,8 @@ Source code and maintenance
 Documentation and website: `https://docs.doubleml.org/ <https://docs.doubleml.org/>`_
 
 DoubleML is currently maintained by
-`@MalteKurz <https://github.com/MalteKurz>`_ and
-`@PhilippBach <https://github.com/PhilippBach>`_.
+`@MalteKurz <https://github.com/MalteKurz>`_, `@PhilippBach <https://github.com/PhilippBach>`_ and
+`@SvenKlaassen <https://github.com/SvenKlaassen>`_.
 
 The source code is available on GitHub: `Python source <https://github.com/DoubleML/doubleml-for-py>`_ and
 `R source <https://github.com/DoubleML/doubleml-for-r>`_.

doc/release/release.rst

@@ -7,6 +7,7 @@ Release notes
 
     **DoubleML 0.6.0**
 
+    - **Release highlight:** Heterogeneous treatment effects (GATE, CATE, Quantile effects, ...)
     - Add out-of-sample RMSE and targets for nuisance elements and implement nuisance estimation 
       evaluation via ``evaluate_learners()``.
       `#182 <https://github.com/DoubleML/doubleml-for-py/pull/182>`_