doc: add specific/multi mount point configuration

[acateland]

Add some documentation on how to add redux-undo to a specific part of a redux state tree, and/or apply different redux-undo configuration to differents redux state tree mount point.

[omnidan]

@acateland thanks a lot for the PR and all your work! 😁

if we still want to add this to the README, I would make a new FAQ section at the end (before the "What is this magic?" section). Then also rename the header to be a question, e.g. "How do I use redux-undo for only a part of my tree?".

Otherwise, I'd simply make a separate FAQ page on the website and refer to that from the README. I don't want the README to get too long, basically it should only contain essential information and link to the website for more information.

[omnidan]

I will merge this for now, we can always rework it later. Thanks!