Include README.md in package metadata (fix #110)

[chambln]

Uses the method described in the Python Packaging User Guide:

https://packaging.python.org/en/latest/guides/making-a-pypi-friendly-readme/#including-your-readme-in-your-package-s-metadata

You can see the result here:
https://test.pypi.org/project/genanki-test-chambln/

Just upload with setup.py and twine as normal:

rm -rf dist/*
python3 setup.py sdist bdist_wheel
python3 -m twine upload dist/*

Or pass --repository testpypi to upload to test.pypi.org instead:

python3 -m twine upload --repository testpypi dist/*