Add doc about state values

[mumoshu]

Extracted from #647 (comment)

[rozcietrzewiacz]

This is pretty critical. Right now only the main README mentions them briefly in a few places, without any clear indication of what they are and how they relate to environment values and release (chart) values.

It looks to me that the entire documentation is lacking structure, starting from key definitions. An impressive number of features and random notes landed in the main README bucket, yet the basics are hard to find. The effort of organizing documentation looks like quite an epic.

Since I'm developing my own chart management standard based on helmfile, I'd be happy to contribute to the documentation if time permits. @mumoshu If you have any thoughts on reorganizing the documentation, let me know.

[mumoshu]

@rozcietrzewiacz Hey! Thanks for the feedback.

Yeah, I believe the great documentation is currently lacking and worth a full restructuring.

As I'm not a good writer I don't have a big picture about where we should head, but I'd definitely review it and comment back if anyone can start the work!