ITv2 protocol details

[BrianHumlicek]

I have added a comprehensive document describing the ITv2 protocol at docs/TLink-ITv2-Protocol.md

I have read through it and it represents my current understanding of things. Bear in mind, this was gleaned mostly from reverse engineering effort, so things like naming (ITv2, TLink) may not align with official internal DSC architecture but I gave it my best guess. I think its pretty complete and I don't expect much to change from here, but you never know what I might discover.

Brian