Add comments about bin/lib separation

bin/gdk

 #!/usr/bin/env ruby
 require 'fileutils'
+# Gitlab Development Kit CLI launcher
+#
+# Note to contributors: this script must not change (much) because it is
+# installed outside the gitlab-development-kit repository with 'gem
+# install'.
+
 def main
   case ARGV.first
   when 'init'

lib/gdk.rb

+# GitLab Development Kit CLI parser / executor
+
 module GDK
   PROGNAME = 'gdk'
+  # This function is called from bin/gdk. It must return true/false or
+  # an exit code.
   def self.main
     case ARGV.shift
     when 'run'