Updated readme to reflect Ubuntu installation

[scotjam]

Having recently attempted installation on Ubuntu 12.04, I found these additional installation requirements shown above - python build packages from apt-get, plus a few packages from pip - PyYAML, argh (newest versions don't work but 0.8.1 does) and possibly also pathtools.

[scotjam]

Thanks for approving this rubyruy (it took me a little while to figure out what was missing so I hope this can help others). I can't see an option to actually commit this change from my side to the master branch though (I think only someone with push rights, ie you, can commit the change?). Thanks also for developing this tool in the first instance!