Add support for RHEL/centos8

Link the doc and describe what is wrong with it.

Docs on how to Install GitLab Runner using the official GitLab repositories was updated in 2017. It is 2020. There are new linux distributions, such us Centos 8 and 8.1 and there is no information about if they are supported by the runners or not.

If it is possible to install runners on them, please add such info and repository links. If it is not possible, please write it clearly, and possibly state if it is work in progress or impossible at all.

Best regards, Szymon Wilkołazki