Update README to talk about Cython

Nelson switched setup.py from Pyrex to Cython a while ago, but didn't update the README. This updates the README. (The work is on top of https://github.com/ebroder/python-zephyr/pull/4 -- if you don't want those changes, I can rebase appropriately or something.)

At the moment, the README claims that Debian packaging for the Cython-using version exists, which isn't true. I'd suggest making a new release with a new version number (1.0?) and updated Debian packaging -- I know I'd like to take some of the recent changes on scripts. (I'm happy to submit another pull request with updated Debian packaging and an updated version, if that sounds reasonable.)