Document how to customize from existing Auto-DevOps.gitlab-ci.yml

Provide documentation on how to customize the existing vendored Auto-DevOps.gitlab-ci.yml

From ~Configuration retrospective, some potential problems that this could solve :

1. Users do not know where the vendored Auto-DevOps.gitlab-ci.yml is
2. Users do not know what the vendored Auto-DevOps.gitlab-ci.yml can support
3. There is no example, similar to the Auto DevOps QuickStart guide on how to copy the Auto-DevOps.gitlab-ci.yml into their project's gitlab-ci.yml and customize that to an example project. (e.g. Make a gitlab-ci.yml for your Rails project based on Auto Devops!)