The documentation is a bit limited currently but wanted to share how to add an External Content Type in Visual Studio.
Starting with a standard SharePoint 2013 App Project
Setup your App settings (SharePoint-hosted for local)
Right-click the project and select Add. There is an extra menu-item called Content Types for an External Data Source.
Put in the OData service URL
Here I am using a free feed from the Azure Datamarket.
Here are the available data entities from this service
After selecting the Event data entity the new list is setup as an External Content Type.