Update documentation

Signed-off-by: Carlos Castro <[email protected]>

docs/services/github.md

@@ -76,6 +76,7 @@ template.app-deployed: |
       logURL: "{{.context.argocdUrl}}/applications/{{.app.metadata.name}}?operation=true"
       requiredContexts: []
       autoMerge: true
+      reference: v1.0.0
     pullRequestComment:
       content: |
         Application {{.app.metadata.name}} is now running new version of deployments manifests.
@@ -89,3 +90,4 @@ template.app-deployed: |
   Setting this option to `false` is required if you would like to deploy older refs in your default branch.
   For more information see the [Github Deployment API Docs](https://docs.github.com/en/rest/deployments/deployments?apiVersion=2022-11-28#create-a-deployment).
 - If `github.pullRequestComment.content` is set to 65536 characters or more, it will be truncated.
+- Reference is optional. When set, it will be used as the ref to deploy. If not set, the revision will be used as the ref to deploy.