Provide guidance on configuring error printer for handling errors

As of Clojure 1.10.1-beta1, errors in non-REPL environments are handled with `ex-str`, which is nice because `ex-str` in turn calls `explain-out` which is user-configurable.

However, there is not clear guidance on how best to configure `s/explain-out` such that this configuration will be run before other namespaces are loaded. Perhaps 'user.clj' is the correct place for this, but will that always be loaded for all non-REPL tasks?

For instance, this code would set up Expound, but right now I'm not sure how to reliably run it before other namespaces load: