Integrate Ğ1 monetary license

Integration to benefits all Python programs

• Integrate Ğ1 monetary license as a git subtree:
  • squash?
• Integrate g1_monetary_license folder into the Python package:
  • Add g1_monetary_license folder to the pyproject.toml
  • Add __init__.py in g1_monetary_license folder (will be in the git history of duniterpy repository, shouldn't be an issue to pull g1_monetary_license changes)
• Add helper to get the path of the licenses
• Write documentation in CONTRIBUTING.md on how to proceed:
  • Create link from the README.md
  • Warn to not modify the monetary license here but on its repository
  • Procedure on how to pull changes from g1_monetary_license repository (remote, subtree)