Add a guide for deploying on Shelly Cloud

Add a step-by-step guide for deploying the Rails Girls app on [Shelly Cloud](shellycloud.com).

![put your app online with shelly cloud rails girls](https://f.cloud.github.com/assets/2712383/2251145/35bc8fa8-9d99-11e3-9595-f35fb495feaf.png)