Document deploymentApiVersion for 1.16+ clusters

Mention upcoming support for postgres

Add deprecation caution

doc/topics/autodevops/index.md

@@ -109,6 +109,7 @@ To make full use of Auto DevOps, you will need:
 
   1. A [Kubernetes 1.12+ cluster](../../user/project/clusters/index.md) for the project. The easiest
      way is to add a [new cluster using the GitLab UI](../../user/project/clusters/add_remove_clusters.md#add-new-cluster).
+     For Kubernetes 1.16+ clusters, there is some additional configuration for [Auto Deploy for Kubernetes 1.16+](#kubernetes-116).
   1. NGINX Ingress. You can deploy it to your Kubernetes cluster by installing
      the [GitLab-managed app for Ingress](../../user/clusters/applications.md#ingress),
      once you have configured GitLab's Kubernetes integration in the previous step.
@@ -635,6 +636,30 @@ be pulled again, e.g. after pod eviction, Kubernetes will fail to do so
 as it will be attempting to fetch the image using
 `CI_REGISTRY_PASSWORD`.
 
+#### Kubernetes 1.16+
+
+> [Introduced](https://gitlab.com/gitlab-org/charts/auto-deploy-app/-/merge_requests/51) in GitLab 12.8.
+
+CAUTION: **Deprecation**
+The default value of `extensions/v1beta1` for the `deploymentApiVersion` setting is
+deprecated, and is scheduled to be changed to a new default of `apps/v1` in
+[GitLab 13.0](https://gitlab.com/gitlab-org/charts/auto-deploy-app/issues/47).
+
+In Kubernetes 1.16 onwards, a number of [APIs were removed](https://kubernetes.io/blog/2019/07/18/api-deprecations-in-1-16/),
+including support for `Deployment` in the `extensions/v1beta1` version.
+
+To use Auto Deploy on a Kubernetes 1.16+ cluster, you must:
+
+1. Set the following in the [`.gitlab/auto-deploy-values.yaml` file](#customize-values-for-helm-chart):
+
+   ```yml
+   deploymentApiVersion: apps/v1
+   ```
+
+1. Set the `POSTGRES_ENABLED` variable to `false`. This will disable Auto Deploy's deployment of PostgreSQL.
+Support for enabling Auto Deploy's deployment of PostgreSQL in a Kubernetes 1.16+ cluster
+is [planned](https://gitlab.com/gitlab-org/charts/auto-deploy-app/issues/28).
+
 #### Migrations
 
 > [Introduced][ce-21955] in GitLab 11.4