Update CONTRIBUTING.md

Author: SvenKlaassen

CONTRIBUTING.md

@@ -79,27 +79,21 @@ $ git fetch upstream
 $ git merge upstream/main
 ```
 
-5. Install the **development dependencies** via
-```bash
-$ pip install -r requirements.txt
-$ pip install -r requirements-dev.txt
-```
-
-6. **Install DoubleML in editable mode** (more details can be found
+5. **Install DoubleML in editable mode** (more details can be found
 [here](https://docs.doubleml.org/stable/intro/install.html#python-building-the-package-from-source))
 via 
 ```bash
-$ pip install --editable .
+$ pip install --editable .[dev, rdd]
 ```
 
-7. **Develop** your code changes. The changes can be added and pushed via
+6. **Develop** your code changes. The changes can be added and pushed via
 ```bash
 $ git add your_new_file your_modified_file
 $ git commit -m "A commit message which briefly summarizes the changes made"
 $ git push origin my_feature_branch
 ```
 
-8. Generate a **pull request** from your fork.
+7. Generate a **pull request** from your fork.
 Please follow our guidelines for pull requests.
 When opening the PR you will be guided with a checklist.