Missing codio-specific instructions

[mverdicchio]

https://github.com/saasbook/hw-rails-intro/blob/master/docs/part_0_A.md

On Codio, students must add the line config.hosts = nil to config/environments/development.rb, change the preview static button to box URL, and run rails server with the command rails server -b 0.0.0.0. These instructions are present (with screenshots) in the former Codio version of the instructions but are missing here.