Define recommended url scheme

[alamers]

As defined in the principles, we would like to separate out the url scheme from the messages themselves in the standard. We would however like to have a recommended url scheme to make it easier for all parties.

There is a separate ticket for creating an example scheme as a temporary solution, but that (or another proposal) should evolve into a recommended url scheme as well.

[cookeac]

The example scheme is done, so replacing this as suggested with #147.