clarify integration with readonly fields

[David Seddon]

I suggest the documentation is adjusted so that it clarifies that in order to show the status as read only, you also need to add the field in the ModelAdmin's readonly_fields. This took me a while to work out (I assumed it would do it automatically).

Thanks for the package though!

[Florent Lebreton]

+1
Thanks for your feedback. Saved me some time !