chore: update README with how to use it with Pundit

[darkaico]

Updating README explaining how to fix an error that will appear if we use Pundit as an authorization system. I used the users resource as an example to follow the documentation guideline

Note

It's common to use Administrate with Pundit (as we could see in the Authorization section in the doc).

[andreibondarev]

@darkaico Is this applicable to this gem or the administrate more broadly?

[darkaico]

Hey @andreibondarev, this error appears when you include this gem (administrate_exportable) in a project that uses pundit as an authentication system (along with the administrate gem, of course).

There should be no errors if you don't have an authentication system.