Update README.md

Just being lazy and asking people to check the wiki ;)  I figure that we're more likely to keep the wiki up to date, at least during early development.  Once things start to settle down then we can put more details in the README.md file, perhaps?  Feel free to change it back! Also, I changed `pip install nilmtk` to `python setup.py install` because setting ourselves up on PyPI takes a little while and we might not get it done before the 15th.