[ci skip] Add missing prologue section of upgrading ruby on rails guide.

Before:

![screenshot 2014-07-01 22 38 25](https://cloud.githubusercontent.com/assets/1000669/3444322/5ff00d98-012d-11e4-80f1-7de096c62944.png)

After:

![screenshot 2014-07-01 22 37 55](https://cloud.githubusercontent.com/assets/1000669/3444312/4ff82efc-012d-11e4-86f1-5f307c081d97.png)