Add cache clearing documentation (!4152) · Merge requests · GitLab.org / GitLab · GitLab

Achilleas Pipinellis - OOO until November 25th (parental leave) requested to merge docs/runners-clear-cache-ee into master Jan 18, 2018

What does this MR do?

Port CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16559