Document how to release pre-releases

In CONTRIBUTING.md document how to:

• release a pre-release on PyPI: append [{a|b|rc}N] to the version, it will be automatically detected as pre-release by PyPI.
• install a pre-release from PyPI:

pip install silkaj --user --upgrade --pre

• install silkaj from PyPI test and the dependencies (i.e. DuniterPy) from PyPI (have been removed from the documentation):

pip install silkaj --user --upgrade -i https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/