Fixed the circup installation instructions in the README. Thanks FoamyGuy!
To use in CircuitPython, simply install the [Adafruit CircuitPython Bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-pcf8574`.
Read the [docs](https://circuitpython.readthedocs.io/projects/pcf8574/en/latest/) for info on how to use it.