Add README.md to MANIFEST.in

The README.md is not listed in MANIFEST.in, meaning that it is not
copied into source distributions.  Since it is subsequently read by
setup.py to set the long_description, this renders source
distributions uninstallable.  This change corrects the problem by
adding README.md to the MANIFEST.in.
Merge pull request #34 from klmitch/add-readme

Add README.md to MANIFEST.in