Show supported python versions in readme

[kniklas]

Below example from: https://github.com/mgedmin/check-python-versions/blob/master/README.rst

.. image:: https://img.shields.io/pypi/pyversions/check-python-versions.svg
    :target: https://pypi.org/project/check-python-versions/
    :alt: Supported Python versions

[kniklas]

Related to #30?