[CentOS-docs] Contribute page

[Jerry Amundson]

The last sentence of http://wiki.centos.org/Contribute
indicates a RHEL Beta is available, but this is not the case,
currently. Maybe a more generic statement, and not specific to release
number, could be in place here? If such a link to "upstream" can be
found of course, as I seem unable to find such things tonight. Not
even a fitting mailing list or "group". Frustrating.

jerry