New instructions for CentOS install

Here are the instructions I have developed for installing under CentOS 6.5. They cover a number of Red Hat family specific issues, including the PHP 5.3.3 problem, the need to use yum instead of apt-get, and a number of package-specific details.  Because its rather extensive, I've elected to include it as a separate file, rather than putting them all under a pitfalls section of installation.md.