python: pyproject.toml needs Project description/readme set

The PyPI page is missing a description! https://pypi.org/project/sedonadb/

I believe that setting `readme = "README.md"` will do this (we'll have to add/improve the README too, since that will show up on the PyPI page).

https://github.com/apache/arrow-nanoarrow/blob/62482b37749ea969ce929eabe2e2a8dc4c3c6726/python/pyproject.toml#L22