Add release notes for pyproject.toml change

Signed-off-by: Juan Luis Cano Rodríguez <juan_luis_cano@mckinsey.com>

RELEASE.md

@@ -16,6 +16,8 @@
 
 ## Bug fixes and other changes
 * Updated `kedro pipeline create` and `kedro catalog create` to use new `/conf` file structure.
+* Converted `setup.py` in default template to `pyproject.toml` and moved flake8 configuration
+  to dedicated file `.flake8`.
 
 ## Documentation changes
 * Update example of using generator functions in nodes.