Document contribution process

CONTRIBUTING.md −> doc/development.md

Install with poetry
Code formatting with black, pre-commit usage
dependency import with flake8 (add it as a dependency).
DuniterPy editable
dev branch
Rework release_packages_on_pypi-fr.md in English and transfert to contributing.md
…

Edited Sep 10, 2019 by Moul

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information