Describe extensibility

[thomas-fossati]

Provide a description of the extensibility features of CoRIM & CoMID.

Specifically, we need to document:

• CDDL sockets ($$NAME-extension) using the new registries
• extensible type choices ($NAME-type-choice) using CBOR tags

Besides, we need to define what we mean by "standard CoRIM type" (see the Profile Types section).

[nedmsmith]

Since this is included in the CoRIM text, it shouldn't be considered an enhancement.

[yogeshbdeshpande]

@nedmsmith @thomas-fossati I think this issue has been addressed. Please revisit and close this!