Document the external URL configuration for the PDF viewer

[aubincleme]

#15 introduced the notion of "Trusted domains" for loading PDFs based on external URLs, but it turns out that this feature is not documented on https://store.xwiki.com/xwiki/bin/view/Extension/PDFViewerMacro#documentation, which makes it difficult for users to know that such configuration exists, and understand how to use it.