Add guidelines on how to choose between an app or a service

[thieman]

We haven't done a great job of explaining why any particular process should be defined as a Dusty app rather than a Dusty service. I believe in general that an app can do everything a service can do and more, so you should prefer a service unless you specifically need the capabilities offered by apps. You should use an app if you are:

1. Editing the code used to run the container's process
2. Relying on host forwarding
3. Relying on conditional links
4. Using pre-defined scripts
5. Running tests

We should organize this information, add anything I've missed, and find a good place for it in the docs.