helpful note on appraisal recovery

Author: jrochkind

README.md

@@ -99,6 +99,14 @@ We use [appraisal](https://github.com/thoughtbot/appraisal) to test ActiveRecord
     $ bundle exec appraisal rails-52 rake test
     $ bundle exec appraisal rails-60 rake test
 
+If you run into trouble with appraisal's gemfiles getting out of date and bundler complaining,
+try:
+
+   $ bundle exec appraisal clean
+   $ appraisal generate
+
+That may make changes to appraisal gemfiles that you should commit to repo.
+
 License
 -------