README.rst: Fix "Title underline too short" error

Fix "Title underline too short" error that is causing PyPI to render the description as plain text rather than RST.

This is an annoying aspect of PyPI -- see https://bitbucket.org/pypa/pypi/issue/214
Merge pull request #2 from msabramo/patch-1

README.rst: Fix "Title underline too short" error