Update docs to show how to install from PyPI without speedups

[dhermes]

From #145:

BEZIER_NO_EXTENSIONS=True .../bin/python -m pip install bezier==0.9.0 --no-binary=bezier

There is mention of

$ BEZIER_NO_EXTENSIONS=True .../bin/python -m pip install .

in DEVELOPMENT.rst but README.rst (and the related files such as docs/index.rst) should probably be updated.