Missing reference to document outside main repository

[mfncooper]

In the v1.7 User Guide, at the top of the section on "Radio Channel – Push to Talk (PTT)", there is a note that states:

A separate document, “Radio-Interface-Guide.pdf ,” goes into much greater detail with examples.

Unfortunately, what is not stated here is that this document is not included in the standard Direwolf documentation, but is part of a separate documentation repository. It would be very helpful to include a note about where people can find this document, since it's somewhat baffling to look within your local Direwolf installation, not find it, and not know where else to look.