[example] Add a readme for rails 4.2

Author: kattrali

example/rails-42/README.md

@@ -0,0 +1,9 @@
+# rails 4.2 example
+
+## Running the app
+
+* Set your API key in `config/initializers/bugsnag.rb`
+* Install the dependencies: `bundle install`
+* Migrate the database: `bundle exec rake db:migrate`
+* Run the app: `bundle exec rackup`
+* Navigate to a web browser and open `localhost:9292` to see an error