Data Package should reference CommonMark

[nichtich]

What Data Package version are you using?

v2

Describe the Issue

Data Package specification requires Markdown in property description and links to http://commonmark.org/ so

• the word "markdown" should be replaced by CommonMark
• the CommonMark specification should be added as reference (this is an independent issue Add formal references #1063)

Participation

• I am willing to submit a pull request for this issue.

[peterdesmet]

That makes sense. I didn't know there was a strongly-defined Markdown spec.