FSMModelMixin feature is not documented

[peterfarrell]

FSMModelMixin is not documented in the README. The mixin appears to fix issues with refresh_from_db and has unit tests. The following note/warning about refresh_from_db is already in the README:

Note that calling refresh_from_db on a model instance with a protected FSMField will cause an exception.

This feature was added in the original source repo as PR-246 which fixed #174 - Model.refresh_from_db doesn't work with protected option.

[pfouque]

That's a good point!
Indeed, it was merged (with a non-engaging comment) just before the original project was closed, and wasn't documented.

It's a nice intention but, IMO the story is not complete as it's only excluding the "state" field from the refresh.
Meaning it will refresh all fields except the one you probably expect. If we document it, that needs to be crystal clear.

@peterfarrell I can see you reviewed that PR, do you want to contribute to the documentation too?

NB: The more I use that library, the less I'm using "protected" fields (I'm just setting editable=False), as "We are all consenting adults here"