HUD for showing current keybindings

I have just started using MM -- great stuff so far -- and like the option to have the Gmail KeyBindings set (more or less), and to add our own KeyBindings, which I have started to do. But then I thought "wouldn't it be great if there could be a Help menu item, or a HUD, to show the current set of active KeyBindings, like you get when you press ? in Gmail. I realise that some KBs would probably be too difficult to summarise, but simple adds, removes, moves, toggles etc. should work?

Thanks.