Update index.md.... add instructions on how to add a `values.yaml` override...

Update index.md.... add instructions on how to add a `values.yaml` override file to the default auto-deploy-app helm chart by setting `HELM_UPGRADE_EXTRA_ARGS` to `--values my-values.yaml` (or `-f my-values.yaml`).

doc/topics/autodevops/index.md

@@ -316,7 +316,7 @@ If a project's repository contains a `Dockerfile`, Auto Build will use
 If you are also using Auto Review Apps and Auto Deploy and choose to provide
 your own `Dockerfile`, make sure you expose your application to port
 `5000` as this is the port assumed by the
-[default Helm chart](https://gitlab.com/gitlab-org/charts/auto-deploy-app).
+[default Helm chart](https://gitlab.com/gitlab-org/charts/auto-deploy-app). Alternatively you can override the default values by [customizing the Auto Deploy helm chart](https://docs.gitlab.com/ee/topics/autodevops/index.html#custom-helm-chart)
 
 #### Auto Build using Heroku buildpacks
 
@@ -452,7 +452,7 @@ be deleted.
 
 Review apps are deployed using the
 [auto-deploy-app](https://gitlab.com/gitlab-org/charts/auto-deploy-app) chart with
-Helm. The app will be deployed into the [Kubernetes
+Helm, which can be [customized](https://docs.gitlab.com/ee/topics/autodevops/index.html#custom-helm-chart). The app will be deployed into the [Kubernetes
 namespace](../../user/project/clusters/index.md#deployment-variables)
 for the environment.
 
@@ -514,7 +514,7 @@ Auto Deploy doesn't include deployments to staging or canary by default, but the
 enable them.
 
 You can make use of [environment variables](#environment-variables) to automatically
-scale your pod replicas.
+scale your pod replicas and to apply custom arguments to the helm deploy command. This is an easy way to [customize the Auto Deploy helm chart](https://docs.gitlab.com/ee/topics/autodevops/index.html#custom-helm-chart).
 
 Apps are deployed using the
 [auto-deploy-app](https://gitlab.com/gitlab-org/charts/auto-deploy-app) chart with
@@ -655,6 +655,9 @@ repo or by specifying a project variable:
 - **Project variable** - Create a [project variable](../../ci/variables/README.md#gitlab-cicd-environment-variables)
   `AUTO_DEVOPS_CHART` with the URL of a custom chart to use or create two project variables `AUTO_DEVOPS_CHART_REPOSITORY` with the URL of a custom chart repository and `AUTO_DEVOPS_CHART` with the path to the chart.
 
+You can also make use of the `HELM_UPGRADE_EXTRA_ARGS` environment variable to ovrride the default values in the `values.yaml` file in the [default Helm chart](https://gitlab.com/gitlab-org/charts/auto-deploy-app).
+To apply your own `values.yaml` file to all Helm upgrade commands in Auto Deploy set `HELM_UPGRADE_EXTRA_ARGS` to `--values my-values.yaml` (or `-f my-values.yaml`).
+
 ### Custom Helm chart per environment **[PREMIUM]**
 
 You can specify the use of a custom Helm chart per environment by scoping the environment variable
@@ -761,7 +764,7 @@ also be customized, and you can easily use a [custom buildpack](#custom-buildpac
 | `KUBE_INGRESS_BASE_DOMAIN`   | From GitLab 11.8, this variable can be used to set a domain per cluster. See [cluster domains](../../user/project/clusters/index.md#base-domain) for more information. |
 | `ROLLOUT_RESOURCE_TYPE`     | From GitLab 11.9, this variable allows specification of the resource type being deployed when using a custom helm chart. Default value is `deployment`. |
 | `ROLLOUT_STATUS_DISABLED`   | From GitLab 12.0, this variable allows to disable rollout status check because it doesn't support all resource types, for example, `cronjob`. |
-| `HELM_UPGRADE_EXTRA_ARGS`   | From GitLab 11.11, this variable allows extra arguments in `helm` commands when deploying the application. Note that using quotes will not prevent word splitting. |
+| `HELM_UPGRADE_EXTRA_ARGS`   | From GitLab 11.11, this variable allows extra arguments in `helm` commands when deploying the application. Note that using quotes will not prevent word splitting. **Tip:** you can use this variable to [customize the Auto Deploy helm chart](https://docs.gitlab.com/ee/topics/autodevops/index.html#custom-helm-chart) by applying custom override values with `--values my-values.yml` (or `-f my-values.yaml`). |
 
 TIP: **Tip:**
 Set up the replica variables using a