Whats needed to get this ready for initial release

This library stems from the original PR here: https://github.com/adafruit/Adafruit_CircuitPython_Register/pull/35

@maholli you mentioned wanting to be pinged if anyone started this up. There is renewed effort to get this finalized and added to the bundle inside of it's own repo here.

@kattni as well for heads up.

@tannewt could you have a look at the initial code and let us know what needs to be done in order for this to be considered ready to publish.

A few "custodial" things come to mind for me are:

- The readme file contains a "creating a driver" section that was lifted from `adafruit_register` directly and needs to be update to reference SPI instead of I2C in several places.
- There is no example code for this repo yet. I am still a little unclear on how to use these new classes (I attempted a few times unsuccessfully back when this was in the origin PR). I am willing to work on them and test them out if anyone can point me in the right direction though. I think one could be similar to this: https://github.com/adafruit/Adafruit_CircuitPython_Register/blob/main/examples/register_rwbit.py but I'm unsure of the proper setup to use in order to try it out with a real device.

My understanding of the feedback given on the PR leads to these two additional things needed also:

- new argument in the constructor for polarity. Possibly named and defaulted as `rw_high_write=True`
- another new argument in the constructor for the "Automatic increment value" (unsure on proper name)

Both of these new arguments would need to get added to the constructor, and have their functionality implemented within the code.

Start by reading the README's "creating a driver" section and comparing it with the original PR and the existing register_rwbit.py example. Review the initial code to determine where the polarity and automatic-increment constructor arguments belong, then verify that their behavior, SPI-oriented documentation, and a usable example cover the release requirements described here.

Written by the indexing model from the issue text.