Clarify limitations of the library add add better usage examples to main readme

[vid553]

The main readme needs to be updated for easier usage.

Limitations I came across:

• the ID's of the settings should not be changed when devices are already deployed (because the chip needs to be fully erased if the keys have been changed)
• user_settings_load should be called before set defaults function

Also add function descriptions to the readme, so it is easier to see how the lib should be used (add setting the default, how to reset to defaults, etc).