Updated README.md file

[gabcvit]

I have been facing an issue due to this misunderstanding for days. So I'm adding a comment in it to avoid having other people facing the same problem. Basically the functionality of "auto selecting" is implemented and working good, but on the README.md file it gives the other fellow programmers the idea that the attributes value and itemKey are both doing it the same, only based in different attributes from the items. That unfortunately isn't true, as stated on the issues #265 and possibly #311 . In order to solve my problem, the attribute itemKey was the one that worked, and not value (when inserting a state variable to the value attribute, on iOS it just keeps scrolling back to the first value of the variable, even when overwriting this value inside the onValueChange.

While this issue is not yet solved, it's a good practice to stay transparent with other users that are using this library, and avoid this kind of confusion at any cost.