Add how to run single feature specs
Resolves point 1 of https://gitlab.com/gitlab-org/gitlab-ce/issues/56198
Based on https://gitlab.com/gitlab-org/gitlab-ce/issues/56198#note_130616619 (thanks @mlapierre !)
Resolves point 1 of https://gitlab.com/gitlab-org/gitlab-ce/issues/56198
Based on https://gitlab.com/gitlab-org/gitlab-ce/issues/56198#note_130616619 (thanks @mlapierre !)