setup.py: Include CHANGELOG.rst in sdist

Make `python setup.py sdist` include CHANGELOG.rst in the tar.gz.
Without it, installing from the released tar.gz will fail because
setup.py expects to read CHANGELOG.rst to generate the long description.

/reviewed-by @kirr
/reviewed-on kirr/xlte!2

MANIFEST.in

+include COPYING README.rst CHANGELOG.rst