Update CONTRIBUTING.md (#48)

* Update CONTRIBUTING.md
The project errors with tox3, so users should use tox4.

CONTRIBUTING.md

@@ -9,6 +9,11 @@ tox devenv -e integration
 source venv/bin/activate
 ```
 
+The development setup ships with tox3, you might want to install tox4:
+```shell
+pip install 'tox>=4,<5'
+```
+
 ## Testing
 
 This project uses `tox` for managing test environments. There are some pre-configured environments