add official centos distribution image

Adding the CentOS file with image information.


Please also add the following description for the CentOS listing:


Short Description
Official CentOS 6 Image as of 12 April 2014.

Full Description:
This is the official CentOS image, and will be updated on a regular schedule or as needed for emergency fixes.

centos:latest will always point to the most recent version currently available
centos:6 will point to the latest version of the CentOS 6 tree. 

In case of issues or bugs, please submit a ticket at https://bugs.centos.org