Add additional messages to copier.yml

[lyashevska]

E.g. _message_before_copy, _message_after_copy, _message_before_update and _message_after_update.
Do not forget to move this line to messages:

python-template/README.dev.md

Line 76 in 7d573c5

If your Python package was created successfully, `cookiecutter` will point you to a file

[fdiblen]

Relevant Copier documentation is here