I have a Document Library definition that I added to the solution. I want to include multiple content types for this Document Library: one that inherits from Folder and one that inherits from Document. I did search on Google but found nothing worthy. I also need this to be inside the Visual Studio project. I can add them manually in the interface just fine, but I have to do it each time I deploy the solution.
Assuming that you already have the content types defined in your project you can do like below.
In your schema.xml, set the attribute EnableContentTypes to TRUE on