Add a long_description to setup.py

Currently the `systemd-python` package on PyPI renders with a "Project description" of
"The author of this package has not provided a project description" because the setup.py
has no `long_description` argument.

As a result, the rendering of the project on PyPi appears either immature or not legitimate.

Ths adds in a `long_description` argument which displays the `README.md` as the "Project description"
on PyPI.

Author: gene1wood

setup.py

@@ -79,6 +79,8 @@ def lib(*names, **kw):
 setup (name = 'systemd-python',
        version = version,
        description = 'Python interface for libsystemd',
+       long_description=open("README.md").read(),
+       long_description_content_type="text/markdown",
        author_email = '[email protected]',
        maintainer = 'systemd developers',
        maintainer_email = '[email protected]',