give information on finding help for each command

At the end of the tasks lists, the new line added gives information on
how to find information on all the possible command that can be ran with
`middleman`. Information include options that can be passed to the
command

middleman-cli/lib/middleman-cli.rb

@@ -50,6 +50,7 @@ module Middleman
 
           shell.say 'Tasks:'
           shell.print_table(list, :ident => 2, :truncate => true)
+          shell.say %(\nSee 'middleman help <command>' for more information on specific command.)
           shell.say
         end
       end