Add a description of the "Tubes" use of navigator.connect.

@slightlyoff @dglazkov @jakeleichtling, I'd like to include this use case so that folks don't feel like we're trying to sneak it through. Can you help me describe it correctly?
Why not just use something meaningful, like npm-style semver in the URL?

One way to get this PR landed is by separating concerns. There are two proposals in there: one for actually using ```navigator.connect``` as Tubes, and another for structuring experimental API URLs. Seems like they could just be two paragraphs -- and then we're golden :smiley: