Added clearer error and documentation for I2C communication failures occurring due to the sensor being in working mode.
To use in CircuitPython, simply install the [Adafruit CircuitPython Bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-scd4x`.
Read the [docs](http://circuitpython.readthedocs.io/projects/scd4x/en/latest/) for info on how to use it.