Clearer documentation on use of ADE Extensibilty model

[msandbu]

I am currently working on testing the extensibility model with use of Terraform, but it is not clear to me how the "Connect the image to your environment definition" works.

I have added this as the content of my enviroment.yaml file which is placed on Github which the ADE connects too, however when I try and sync it does not show up.

name: WebApp
summary: Deploys Web App on Terraform Container
description: Deploys an Azure Web App with Terraform
runner: "test.azurecr.io/customimage:1.0.0"

Is this the only content I need besides the README.md file on the github repo? (would love it if you added an example structure with content on your github example list.