Provide clarification on mirrored repositories for pipelines

Description

Provide clarification for mirrored repositories that already have a valid .gitlab-ci.yml

Proposal

Add a blurb about triggers on mirrors after:

Now if you go to the **Pipelines** page you will see that the pipeline is
pending.

Now if you go to the **Pipelines** page you will see that the pipeline is
pending.

Note: If you have a mirrored repository you may need to enable pipeline triggers.

Links / references

https://gitlab.com/help/ci/quick_start/README.md

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/ci/quick_start/README.md

Overview

A customer reported some confusion/misdirection with a mirrored repository where it doesn't report or provide any direct documentation about enabling triggering of a remote repository.

It seems like a quick note on the documentation that it links to would have alleviated the confusion.