Document how to implement incremental rollouts with GitLab CI

Problem to solve

Auto DevOps offers an incremental rollout feature which uses GitLab CI. Customers who are not using Auto DevOps could achieve the same functionality by:

1. Defining incremental stages in the gitlab-ci.yml (mimicking auto devops https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml#L60)
2. Define $INCREMENTAL_ROLLOUT_MODE and $INCREMENTAL_ROLLOUT_ENABLED as environment variables

However, this is not documented outside of Auto DevOps.

Further details

Proposal

Document something along the lines of: How to perform incremental rollouts with GitLab CI

Who can address the issue

We'll need some CI experts, not sure who would be best but perhaps @jlenny can point us in the right direction

Other links/references