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.

- [x] 1. Is the categorization of transitions helpful?
- [x] 2. Should this include the concept of curies or prefixes?
- [x] 3. Is the concept of anonymous links and actions helpful?
- [x] 4. Does it make sense to include embedded resources under transitions?
- [x] 5. How do we address more complex attributes? Ones that may include arrays of objects in JSON or nested elements in XML?

Once we come to a consensus, we can merge it into the repo. Thanks!
I've decided to leave this as-is. Reason being, a transition only becomes a safe/unsafe transition once it is resolved. If you all still feel it's good to explain, it feels like there should be four categories.

1. Untemplated, safe
1. Templated, safe
1. Untemplated, unsafe
1. Templated, unsafe

Seems like it's more confusing to draw such distinctions.