add mention of environment variable management feature in Control Center docs

Author: sosa-vaadin

articles/control-center/application-deployment/index.adoc

@@ -131,6 +131,14 @@ Once all fields are completed, click [guibutton]*Deploy*.
 Control Center now deploys your application to the Kubernetes cluster, creates the necessary resources, and handles configurations such as load balancing and service discovery.
 
 
+=== Environment Variables
+
+Environment variables and their values can be set to applications deployed with Control Center.
+These variables can be managed by clicking the [guibutton]*Environment Variables* button.
+This opens a dialog to add, edit, or remove environment variables.
+Some environment variables are considered essential and cannot be modified or removed.
+Making any changes to environment variables triggers a re-deploy to make the new values available.
+
 == Accessing Application
 
 Once the deployment is complete, your application can be accessed using the hostname you specified during deployment. For example, if you deployed to a cloud environment you'd use something like, `https://app.example.com`. If you're working in a local environment, you'd use `http://app.local`, instead.