Add notice about HTTPS cloning for CI
What does this MR do?
Adds a notice about potential problem with GitLab CI unable to clone sources: https://gitlab.com/gitlab-org/gitlab-ce/issues/22484#note_16612187
Adds a notice about potential problem with GitLab CI unable to clone sources: https://gitlab.com/gitlab-org/gitlab-ce/issues/22484#note_16612187