[ci skip] Add Warnings for change_column_null and change_column_default ...

...in guides
What I wanted in this is that I should be able to add the code chunk under the heading of WARNING, which I am not able to do?
Is it fine to specify warning first..Please See the screenshot to understand what I want here.
![guide-issue](https://cloud.githubusercontent.com/assets/1955930/3241543/62e9655c-f13e-11e3-8cc2-633c60688d07.png)