Document mode

docs/src/pages/application.md

@@ -3,3 +3,16 @@
 An @:api(krop.Application) in what a Krop [server](server.md) runs, and thus building an `Application` is the end goal of working with Krop.
 
 An `Application` consists of any number of [Routes](routes/README.md) followed by a catch-all that handles any requests not matched by a route. The usual catch-all is @:api(krop.tool.Application.notFound).
+
+
+## Development and Production Mode
+
+Krop can run in one of two modes: development and production. In development
+mode it shows output that is useful for debugging and otherwise inspecting
+the running state. In production this output is hidden.
+
+The mode is set by the `krop.mode` JVM system property. If it has the value of
+"development" (without the quotes; any capitalization is fine) then the mode
+is development. Otherwise it is production.
+
+The mode is determined when Krop starts, and is available as the value of `krop.Mode`.