Add README to pypi page

[MartinThoma]

Currently, it looks like this: https://pypi.org/project/flake8-logging-format/

The change would use the currend README.md to be displayed there. A new update of the plugin on pypi is necessary for this to be active.

Example where you can see this:

• PyPI: https://pypi.org/project/mpu/
• Github: https://github.com/MartinThoma/mpu