How to include docs using distutils?

[skip]

I have a simple setup.py file:

#!/usr/bin/env python

from distutils.core import setup
setup(name='lockfile',
author='Skip Montanaro',
author_email='(e-mail address removed)',
maintainer='Skip Montanaro',
maintainer_email='(e-mail address removed)',
url='http://www.webfast.com/~skip/python/',
download_url='http://www.webfast.com/~skip/python/lockfile.tar.gz',
version='0.1',
py_modules=['lockfile'],
data_files=[('doc', ['lockfile.rst'])],
)

I've used the data_files arg to setup to note where the docs live. Alas,
the sdist command doesn't include this in the tar file:

% tar tfz dist/lockfile-0.1.tar.gz
lockfile-0.1/
lockfile-0.1/lockfile.py
lockfile-0.1/PKG-INFO
lockfile-0.1/README
lockfile-0.1/setup.py

I'm a complete distutils novice. I can't for the life of me understand why
the doc directory isn't included in the tar file. As far as I can tell
distutils has no explicit provision at all for distributing documentation,
which seems very weird. What am I missing?

Thanks,

Skip