Add link to CentOS 7 install guide.
After a cup of coffee, should really make the CentOS 7 guide a separate page, which has been done, so undoing my changes here.
This reverts commit
89328822305dcac5ea26e168ece5407bee002ef0
Modified to match docs.openafs.org/QuickStartUnix/HDRWQ50.html#idm45731875295408 which is verified to be necessary on a stock CentOS 7 system