Add note about OS X firewall

Author: brittag

sites/en/installfest/create_a_rails_app.step

@@ -27,6 +27,8 @@ step "Create a new Rails app" do
   console "cd test_app"
   console "rails server"
 
+  tip "In OS X, you may need to let Ruby accept incoming network connections through your firewall.  Select 'allow' in the pop up."
+
   tip "In Windows, you may need to let Ruby and Rails communicate through your firewall.  Say yes to the pop up."
 
   tip "Shortcut: Just type 'rails s'" do