docs: Add transparent history of reasoning for proposed 0.6.0 model changes (#289)

Just documenting the proposed changes for 0.6.0 with their reasoning,
currently just for DCC and core.

You can view the markdown directly, if that's easier, at:


https://github.com/absoludity/spec-untp/blob/add-0.6.0-reasoning/docs/untp-data-model/changes-v0.6.0-alpha1.md

https://github.com/absoludity/spec-untp/blob/add-0.6.0-reasoning/docs/untp-data-model/changes-v0.6.0-alpha2.md

~~I'll do separate PRs with the DPP and friends for review.~~ This PR
contains the suggested changes for all models.

Author: absoludity

docs/untp-data-model/README.md

@@ -0,0 +1 @@
+Currently the release process doesn't have a transparent identification of changes between UNTP versions nor the reasons for those changes. Until we get a chance to update the process to include this, the files in this directory serve to make this transparent for our future selves working on the data model.