Add special installation tips for CentOS 5 users

Add special installation tips for CentOS 5 users
CentOS 5 was heavily used in today's enterprise environment.
So we need to tune our document friendly for our CentOS users.
I was one of the CentOS 5 users, we have more than 50 servers running . 
Really thanks composer for helping us manage the dependency, but the installation should be more clear and easily to satisfied with pure CentOS 5 envrionment.

Tested under CentOS 5.9