Add sequence diagrams

[mmccool]

To graphically explain discovery process, for both directory and peer-to-peer discovery, add sequence diagrams as in https://github.com/w3c/wot-architecture/blob/master/images/message-flows/lifecycle-2.svg
See also the tools used to generate these from textual descriptions in the TD spec.

[mmccool]

Since plantUML has been used to talk about Introduction mechanisms we should keep using that. We need additional diagrams for overall process and also need to update the diagrams in Architecture to be consistent.
See https://github.com/w3c/wot-discovery/tree/master/images

[mmccool]

I have some experience using PUML now and this would be helpful. However, since all diagrams are informative, we can defer to PR; it's not a blocker for CR. I would in general we difficult to do something too specific since there are so many different introductions. But perhaps we could do a "typical" flow.

[mmccool]

need to defer to next version