[dxwg] PROF roles and their definitions (#1049)

kcoyle has just created a new issue for https://github.com/w3c/dxwg:

== PROF roles and their definitions ==
A small set of roles has been included in the PROF document as a starting set. That set is:

- constraints
- example
- guidance
- mapping
- schema
- specification
- validation
- vocabulary

There are questions about the definitions in a [Google doc](https://docs.google.com/document/d/1ddygq4EcKr1DzJykdhM_WxkkmTAoU1qQWsf8xuZxcKc/edit) that are still not addressed and may need additional discussion. Also, we need a general consensus on whether these are the roles that the group feels are the basic ones to be included. 

Please view or discuss this issue at https://github.com/w3c/dxwg/issues/1049 using your GitHub account