Add deprecation for only/except in Auto DevOps

Links to the issue where we plan to perform the change to rules.

doc/topics/autodevops/customize.md

@@ -218,6 +218,16 @@ include:
 
 See the [Auto DevOps template](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml) for information on available jobs.
 
+CAUTION: **Deprecation**
+Auto DevOps templates using the [`only`](../../ci/yaml/README.md#onlyexcept-basic) or
+[`except`](../../ci/yaml/README.md#onlyexcept-basic) syntax will switch
+to the [`rules`](../../ci/yaml/README.md#rules) syntax, starting in
+[GitLab 13.0](https://gitlab.com/gitlab-org/gitlab/-/issues/213336).
+If your `.gitlab-ci.yml` extends these Auto DevOps templates and override the `only` or
+`except` keywords, you must migrate your templates to use the
+[`rules`](../../ci/yaml/README.md#rules) syntax after the
+base template is migrated to use the `rules` syntax.
+
 ## PostgreSQL database support
 
 To support applications requiring a database,