Document use case of the PdfViewer control

[pvginkel]

A number of issues and pull requests have been created to extend the toolbar of the PdfViewer control. However, these should not be applied. The reason for this is that the PdfViewer control really is not meant to be used for advanced use cases.

The purpose of the PdfViewer control is to have a fast starting point to get up and running. However, most people will want to customize their experience and have control over what kind of functionality they want to expose. Because of this they should be using the PdfRenderer control. This control has the same functionality as the PdfViewer control, except that it does not have a toolbar. The PdfViewer control can still be used as a starting point, e.g. by copy and pasting the code into your own project. That way you can get the exact functionality you desire.

The primary problem with the existence of the PdfViewer control is that people do use this and changing it will break their application, because they expect the buttons that are there now to stay there. Looking back the PdfViewer control should never have been added to the project.

This information should be documented in the main README file.