[CentOS-docs] contrib repository not mentioned on AdditionalResources/Repositories

[Sean Gilligan]

The 'contrib' repository is mentioned on FAQ/General #13:

http://wiki.centos.org/FAQ/General#head-92ba307b036080a16dbffa44486cde25c081fcca

but not on the AdditionalResources/Repositories page:

http://wiki.centos.org/AdditionalResources/Repositories