Improve contributing docs.

[bellisk]

I had trouble getting started because of this, so adding it for future contributors.

[mi-wood]

Thanks for the update and sorry you had trouble getting this running!

This looks to be a change in rails 4.2 where rails s defaults to 127.0.0.1 (https://stackoverflow.com/questions/27799260/rails-4-2-server-port-forwarding-on-vagrant-does-not-work).

Since this is a single use VM, it's entirely reasonable to bind to 0.0.0.0 instead of 10.0.2.15. This will be resilient to and sort of vagrant networking change or defaults.

[carolinux]

This worked for me:

vagrant ssh
cd /vagrant/
rails s --binding=0.0.0.0