Update documentation regarding running test

CONTRIBUTING.md

+Contributing
+============
+
 We love pull requests. Here's a quick guide:
 
 1. Fork the repo.
@@ -13,7 +16,6 @@ a test!
 
 5. Push to your fork and submit a pull request.
 
-
 At this point you're waiting on us. We like to at least comment on, if not
 accept, pull requests within three business days (and, typically, one business
 day). We may suggest some changes or improvements or alternatives.
@@ -26,7 +28,37 @@ taken straight from the Ruby on Rails guide:
 * Update the documentation, the surrounding one, examples elsewhere, guides,
   whatever is affected by your contribution
 
-Syntax:
+Running Tests
+-------------
+
+Paperclip uses [Appraisal](https://github.com/thoughtbot/appraisal) to aid
+testing against multiple version of Ruby on Rails. This helps us to make sure
+that Paperclip performs correctly with them.
+
+### Bootstrapping your test suite:
+
+    bundle install
+    bundle exec rake appraisal:install
+
+This will install all the required gems that requires to test against each
+version of Rails, which defined in `gemfiles/*.gemfile`.
+
+### To run a full test suite:
+
+    bundle exec rake
+
+This will run Test::Unit and Cucumber against all version of Rails
+
+### To run single Test::Unit or Cucumber test
+
+You need to specify a `BUNDLE_GEMFILE` pointing to the gemfile before running
+the normal test command:
+
+    BUNDLE_GEMFILE=gemfiles/3.2.gemfile ruby -Itest test/schema_test.rb
+    BUNDLE_GEMFILE=gemfiles/3.2.gemfile cucumber features/basic_integration.feature
+
+Syntax
+------
 
 * Two spaces, no tabs.
 * No trailing whitespace. Blank lines should not have any space.

README.md

@@ -451,13 +451,13 @@ If you'd like to contribute a feature or bugfix: Thanks! To make sure your
 fix/feature has a high chance of being included, please read the following
 guidelines:
 
-1. Ask on the mailing list[http://groups.google.com/group/paperclip-plugin], or
-   post a new GitHub Issue[http://github.com/thoughtbot/paperclip/issues].
+1. Ask on the [mailing list](http://groups.google.com/group/paperclip-plugin), or
+   post a new [GitHub Issue](http://github.com/thoughtbot/paperclip/issues).
 2. Make sure there are tests! We will not accept any patch that is not tested.
    It's a rare time when explicit tests aren't needed. If you have questions
    about writing tests for paperclip, please ask the mailing list.
 
-Please see CONTRIBUTING.md for details.
+Please see `CONTRIBUTING.md` for more details on contributing and running test.
 
 Credits
 -------
@@ -473,4 +473,5 @@ The names and logos for thoughtbot are trademarks of thoughtbot, inc.
 License
 -------
 
-Paperclip is Copyright © 2008-2011 thoughtbot. It is free software, and may be redistributed under the terms specified in the MIT-LICENSE file.
+Paperclip is Copyright © 2008-2011 thoughtbot. It is free software, and may be
+redistributed under the terms specified in the MIT-LICENSE file.

RUNNING_TESTS.md

+Running Tests
+=============
+
+Please see `CONTRIBUTING.md` in "Running Tests" section for more information.