minor README modification (dependency

[Chaztikov]

Just FYI: I also needed to install pybind11, phat, cechmate

Installing collected packages: pybind11, phat, cechmate Running setup.py install for phat ... done Successfully installed cechmate-0.0.8 phat-1.5.0 pybind11-2.5.0

[maminian]

Interesting, I thought that a pip install of scikit-tda amounted to installing its four sub-packages? Thanks for pointing those out. I'm not very disciplined using environments, so it's easy to miss that kind of thing!