Add doc for cli package

main.go

@@ -1,3 +1,11 @@
+// mtg is just a command-line application that starts a proxy.
+//
+// Application logic is how to read a config and configure mtglib.Proxy.
+// So, probably you need to read the documentation for mtglib package
+// first.
+//
+// mtglib is a core of the application. The rest of the packages provide
+// some default implementations for the interfaces, defined in mtglib.
 package main
 
 import (