add terminology section

[Xavier Vilajosana]

7. The document could use a section on the terminology used in the document, to provide definitive sources for definitions of the various terms and acronyms used in the document. Most likely, the terminology section would simply point to relevant sections of [IEEE802154] and [I-D.ietf-6tisch-terminology].

TW> Editorial. TW> @Editor, please add a "Terminology" section which points to [I-D.ietf-6tisch-terminology].