Add notice about GitLab Runner to requirements docs

This is related to #14589 (closed), and problems that may stem from running GitLab Runner on same machine user installed GitLab web app on.

@axil Should we also add note about this to CI docs?

I think a note in the Runner docs would make more sense. In CI docs, we already assume users have installed their own Runner or we link them the the Runner installation docs.

Reassigned to @axil

@axil Do you want me to do some additional corrections here?

Added 1 commit:

• 3122e5cc - Add link to Runner security doc

Added 1 commit:

• d86190ee - Add a note about installing Runners in ci/runners/README.md

Enabled an automatic merge when the build for d86190ee succeeds

@grzesiek I pushed some additions to the runners doc. Thanks!

Canceled the automatic merge

Enabled an automatic merge when the build for d86190ee succeeds

Status changed to merged

mentioned in commit fd13cd18