README.md in junk_drawer-1.4.0 vs README.md in junk_drawer-1.5.0

@@ -218,9 +218,16 @@
 To install this gem onto your local machine, run `bundle exec rake install`. To
 release a new version, update the version number in `version.rb`, and then run
 `bundle exec rake release`, which will create a git tag for the version, push
 git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
 
+In order to run tests against different Rails versions, you can use
+`BUNDLE_GEMFILE`:
+
+```sh
+$ BUNDLE_GEMFILE=gemfiles/rails_4.2.gems rake spec
+```
+
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at
 https://github.com/thread-pond/junk_drawer. This project is intended to be a
 safe, welcoming space for collaboration, and contributors are expected to