Minor cleanup, and incorporate feedback as a result of the WebApps F2F:

* Per discussion with @mnot and @AFBarstow, add the public-webapps mailing
  list as a place where this content will be discussed.
* Per discussion with @masinter, add integration with
  draft-thaler-appsawg-uri-scheme-reg to the Todo list.
* Per discussion with @pcotton and @marcoscaceres, add incorporating the
  substance of the informative notes that immediately preceed the [URL]
  reference in the W3C HTML5 standard into this document as a Todo.