Be more explicit in multiple stores docs

[davetapley]

When I was new to MST I glossed over of the same type (not understanding what it meant) in this:

https://github.com/jonbnewman/mobx-store-provider/blob/34788e01a74beb21d418802219898947733d4259/docs/multiple-stores.md?plain=1#L9

Accordingly when I added a new store (definition, not instance) I created a new nested <Provider> (as show on that page). In hindsight I think the RootStore pattern would have been more appropriate, but I wasn't aware of it because that page is titled Multiple stores.

Would you accept a PR to put a ⚠️ on that Multiple stores page that using a single provider and the RootStore approach is better* if you only have one instance of each model type?

* I argue that it's better for this reason.

[jonbnewman]

Absolutely, please feel free to submit a PR.

Thanks!