Documentation: Add an overview that explains and compares the main purchasing paths

I recently misunderstood the in-app purchasing API and that misunderstanding was rooted in the fact that I didn't know there were three distinct and valuable purchasing paths that each require different integration with the APIs. (Maybe there are more?)

In my case, it was confusing the non-receipt, real money path with the receipt, real money path. I even remembered reading about the non-receipt path somewhere.

I think a quick overview of the main in-app purchasing paths, that is easy to find from a Bing or Google search and easy to find from the documentation for the relevant APIs would make getting started a lot easier.

If that already exists, then just making it easier to find.