commands/command: Add a package comment

I'm not quite sure which file in this package is the most prominent,
but command.go seemed like a reasonable choice.

command.go

+/*
+Package commands provides an API for defining and parsing commands.
+
+Supporting nested commands, options, arguments, etc.  The commands
+package also supports a collection of marshallers for presenting
+output to the user, including text, JSON, and XML marshallers.
+*/
+
 package commands
 
 import (