Elements of a Hypermedia Message Document

Per issue #4, this is the first draft of this document describing the elements of a hypermedia message. It is all open for discussion, and I'd be happy to change anything. Below are some discussion items.

- [ ] 1. Is the categorization of transitions helpful?
- [ ] 2. Should this include the concept of curies or prefixes?
- [ ] 3. Is the concept of anonymous links and actions helpful?
- [ ] 4. Does it make sense to include embedded resources under transitions?

Once we come to a consensus, we can merge it into the repo. Thanks!