Add environment variable coverage to build step

Users will need to add pipeline variables during the build module image step if they have any env variables in the module.json file, which the sample project does have.

Author: kln4dt

articles/iot-edge/how-to-ci-cd.md

@@ -90,6 +90,12 @@ In this section, you create a new build pipeline. Configure the pipeline to run
    * **Default platform**: Select the appropriate platform for your modules based on your target IoT Edge device. 
    * **Output variables**: The output variables include a reference name that you can use to configure the file path where your deployment.json file will be generated. Set the reference name to something memorable like **edge**. 
 
+   These configurations use the image repository and tag in the `module.json` file to name and tag the module image. The **Build module images** also helps replace the variables with the exact value you defined in the `module.json` file. In VS/VS Code, you are specifying the actual value in a `.env` file. In Azure Pipelines, you set the value in the Pipeline Variables tab. Move to Variables tab and configure the Name and Value as following.
+
+    * **ACR_ADDRESS**: Your Azure Container Registry address. 
+
+    If you have other variables in your project, you can specify the name and value in this tab. The **Build module images** can only recognize the variables are in `${VARIABLE}` flavor, make sure you are using this in your `**/module.json` files.
+    
 7. Select the second **Azure IoT Edge** task to edit it. This task pushes all module images to the container registry that you select.
 
    * **Display name**: The display name is automatically updated when the action field changes.