centos.md

Hi Jamey,

I created a document that lists all the packages that need to be installed on CentOS to run xbsv, vivado, Bluespec, etc.  From now on, I'll actively maintain this document. Here is a link to it
https://github.com/suhaspai/xbsv/blob/master/doc/centos.md on how it appears at the forked branch.

Thanks,
Suhas

Suhas