Document how to obtain a TOCALL and where to configure it

[sjlongland]

Right now, we're using APZAIO due to the software library's experimental status, but this really should be replaced with an official allocation so people can recognise anything that's aioax25-based.

https://github.com/aprsorg/aprs-deviceid/

[sjlongland]

aprsorg/aprs-deviceid#264

[sjlongland]

As a library and not and end-user application, this tool technically doesn't need its own TOCALL, we'll keep using APZAIO as the default, however I should clarify the documentation regarding how to set this for end-user applications that people build with it.