Add basic documentation about Serverless template

doc/user/project/clusters/serverless/index.md

@@ -114,28 +114,29 @@ Follow these steps to deploy a function using the Node.js runtime to your Knativ
     - Public, continue to the next step.
     - Private, you will need to [create a GitLab deploy token](../../deploy_tokens/index.md#creating-a-deploy-token) with `gitlab-deploy-token` as the name and the `read_registry` scope.
 
-1. `.gitlab-ci.yml`: This template allows to define the stage, environment, and
-   image to be used for your functions. It must be included at the root of your repository:
+1. `.gitlab-ci.yml`: this configuration allows to define the environment to be
+   used to deploy your functions. It must be included at the root of your repository:
 
    ```yaml
-   stages:
-     - deploy
+   include:
+     template: Serverless.gitlab-ci.yml
 
    functions:
-     stage: deploy
-     environment: test
-     image: gcr.io/triggermesh/tm:v0.0.9
-     script:
-      - tm -n "$KUBE_NAMESPACE" set registry-auth gitlab-registry --registry "$CI_REGISTRY" --username "$CI_REGISTRY_USER" --password "$CI_JOB_TOKEN" --push
-      - tm -n "$KUBE_NAMESPACE" set registry-auth gitlab-registry --registry "$CI_REGISTRY" --username "$CI_DEPLOY_USER" --password "$CI_DEPLOY_PASSWORD" --pull
-      - tm -n "$KUBE_NAMESPACE" deploy --wait
-
+     extends: .serverless:deploy:functions
+     environment: production
    ```
 
-    The `gitlab-ci.yml` template creates a `Deploy` stage with a `functions` job that invokes the `tm` CLI with the required parameters.
+    This `.gitlab-ci.yml` creates a `Deploy` stage with the `functions` job
+    that invokes some predefined commands to deploy your functions to Knative.
+
+2. `serverless.yml`: this file contains the metadata for your functions,
+   such as name, runtime, and environment.
 
-2. `serverless.yml`: This file contains the metadata for your functions,
-   such as name, runtime, and environment. It must be included at the root of your repository. The following is a sample `echo` function which shows the required structure for the file. You can find the relevant files for this project in the [functions example project](https://gitlab.com/knative-examples/functions).
+   It must be included at the root of your repository.
+   The following is a sample `echo` function which shows the required structure
+   for the file.
+
+   You can find the relevant files for this project in the [functions example project](https://gitlab.com/knative-examples/functions).
 
    ```yaml
    service: my-functions
@@ -234,32 +235,21 @@ Add the following `.gitlab-ci.yml` to the root of your repository
 (you may skip this step if you've previously cloned the sample [Knative Ruby App](https://gitlab.com/knative-examples/knative-ruby-app) mentioned above):
 
 ```yaml
-stages:
-  - build
-  - deploy
+include:
+  template: Serverless.gitlab-ci.yml
 
 build:
-  stage: build
-  image:
-    name: gcr.io/kaniko-project/executor:debug
-    entrypoint: [""]
-  only:
-    - master
-  script:
-    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
-    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE
+  extends: .serverless:build:image
 
 deploy:
-  stage: deploy
-  image: gcr.io/triggermesh/tm@sha256:e3ee74db94d215bd297738d93577481f3e4db38013326c90d57f873df7ab41d5
-  only:
-    - master
-  environment: production
-  script:
-    - echo "$CI_REGISTRY_IMAGE"
-    - tm -n "$KUBE_NAMESPACE" --config "$KUBECONFIG" deploy service "$CI_PROJECT_NAME" --from-image "$CI_REGISTRY_IMAGE" --wait
+  extends: .serverless:deploy:image
 ```
 
+`Serverless.gitlab-ci.yml` is a template that allows customization.
+You can either import it with `include:` directive and use `extends` to
+customize your jobs, or you can inline entire template by choosing it
+from "Templates" dropdown. (TODO: link to docs about templates).
+
 ### Deploy the application with Knative
 
 With all the pieces in place, the next time a CI pipeline runs, the Knative application will be deployed. Navigate to