Consider changing manual installation instructions to reflect deprecation of setup.py

[alh38]

Just FYI, I've had a number of issues installing liftoff, and thought I was successful with a manual installation as per the current instructions in the readme, but still had issues upon execution. I saw the logs during installation indicated that setup.py install is no longer supported (details here), and after changing python setup.py install to pip install ., it worked much better! Might be worth updating the readme to reflect the deprecation of setup.py.