[CentOS-docs] Pull Request wiki.c.o/AdditionalResources/Repositories

> CentOS repos. There are others that do. Some of the repos that overwrite
> core packages do so with little packages like sqlite (yum uses sqlite so
> changing the version of it is not a Good Thing for system stabilty).
> Other repos in that list have been effectively unmaintained for a number
> of years so they contain packages that may have severe unfixed security
> vulnerabilities.

I suggest that we shouldn't use euphemisms when it's far more valuable
to come out and say it.  I would certainly appreciate if the centos docs
explicitly tagged the other repos with these comments.  Factual commentary
about risks does not come anywhere close to libel...

regards, mark hahn.