Update README.md

yum install rrdtool rrdtool-devel

For centos5, we will get some troubles like that [rrdtool version is old](https://gist.github.com/sonots/8661761), and [sqlite version is old](http://blog.livedoor.jp/sonots/archives/29328032.html), so I noted as RHEL/CentOS 6.x