Note the required appraisal step for installation

I'm working on fixing a bug just now, and the steps in CONTRIBUTING.md weren't quite complete: running `bundle && rake` fails because this project requires the appraisal setup. It took me five minutes to find out how to do this.

So I've updated the documentation to include the `bundle exec rake appraisal:install` step, and also to change `rake` to `bundle exec rake`.