Provide example in the readme for testing organizers

[thisismydesign]

Hey,

I saw there're closed issues asking about how to test organizers (https://github.com/collectiveidea/interactor/issues/161) and I also found this: https://stackoverflow.com/questions/43984783/how-do-we-test-interactor-organizers-using-rspec

If you agree that this is the way could you include an example in the readme, or would you accept a pr about it?