setup.py: add long_description (#1073)

twine (the tool we use to upload packages to PyPI) is currently failing if the long_description (used to render a project's page on PyPI website) is not set. Somehow it complains that it is not well formatted reStructuredText, but it's simply empty...
This looks like a bug, or bad interaction between twince and setuptools, because the field is technically optional.
Also see https://github.com/pypa/twine/issues/960 and https://github.com/pypa/twine/issues/908.

This issue is currently preventing the upload of newly built Brotli v1.1.0 Python wheels to PyPI:
https://github.com/google/brotli-wheels/issues/18#issuecomment-1706910190

Anyway, we may well set the long_description to the content of the README.md file, as it's customary for python projects.