Use `python -m` pip in README

[dstansby]

The pip documentation recommends using python -m pip instead of just pip to avoid the errors mentioned in the README of this appendiex. This PR updates the instructions to use python -m pip in place of pip, and removes the now-redundant appendex section.

[rweigel]

For future reference, this is the explanation I should have referred to in the appendix: https://www.anaconda.com/blog/using-pip-in-a-conda-environment