Specify a way how to add a local / custom stand-in

[marenamat]

If the model defines its own compound data, it may as well make use of some private tags or experimental values. We should at least outline how to do this.

[cabo]

Right. Is that something we need to do in this document?
It seems to me stand-ins need to be made ready-to-use using the approaches defined in the current document, and we can always add a YANG extension later to cover the development of new stand-ins.
There should be no requirement to touch the models in order to be able to define a new stand-in tag.