Document templates

In https://github.com/sampsyo/confit/issues/2, we made a significant change to how Confit is used: extensible _templates_ have replaced ad-hoc validation. This needs documentation.

One nice observation about the current system: you can fluidly trade off the granularity at which you do validation. You can validate the entire configuration at once and use only valid values later, or you can validate every option individually (as in "old" Confit).