Add Documentation for custom Kconfig

[onsmallfin]

The readme only mentions customizing the keymap, but as I understand there is the option to customize the kconfig.
In particular I would like to enable/ disable only specific options such as HID_POINTING. From the README and by looking at the code here and the zmk repo I was not able to deduce how I would go about that.

What makes me a bit weary to just try anything is that there are some kconfig options that are better left untouched such as CONFIG_ZMK_RGB_UNDERGLOW_BRT_MAX, as they could break stuff: https://github.com/moergo-sc/zmk/blob/faf11f929a65a12a688b481cbe333eda1fd83a77/app/boards/arm/glove80/glove80_rh_defconfig#L62

I think the readme should clear up:
Is the configuration provided in config/glove80.conf meant to be a full config that completely substitutes the default konfig or is it merged into the default configuraton and in case of conflicts the values supplied in config/glove80.conf are used?

If someone were to clear that up, I am more than happy to prepare a PR and add more info to the readme.

Thank you very much :)