Add toml and tomli dependencies to CONTRIBUTING

toml is needed to create min_requiremenst file
tomli is needed to parse pyproject configuration file.
Without tomli I see warning:
`configuration file was ignored, because the `tomli`
package is not installed`

Signed-off-by: ela-kotulska-frequenz <[email protected]>

Author: ela-kotulska-frequenz

CONTRIBUTING.md

@@ -21,7 +21,7 @@ python -m pip install -e .
 You can also use `nox` to run the tests and other checks:
 
 ```sh
-python -m pip install nox
+python -m pip install nox toml tomli
 nox
 ```