Add some instructions to the README

README.md

@@ -2,4 +2,14 @@
 
 This is the sample application for
 [*Ruby on Rails Tutorial: Learn Web Development with Rails*](http://railstutorial.org/)
-by [Michael Hartl](http://michaelhartl.com/).
+by [Michael Hartl](http://michaelhartl.com/). You can use this reference implementation to help track down errors if you end up having trouble with code in the tutorial. In particular, as a first debugging check I suggest getting the test suite to pass on your local machine:
+
+    $ cd /tmp
+    $ git clone git@github.com:railstutorial/sample_app_2nd_ed.git
+    $ cd sample_app_2nd_ed
+    $ bundle install
+    $ bundle exec rake db:migrate
+    $ bundle exec rake db:test:prepare
+    $ bundle exec rspec spec/
+
+If the tests don't pass, it means there's something wrong with you system. If they do pass, then you can debug your code by comparing it with the reference implementation.