SELECT is select, and START is start. (on iOS 14)

[ToddLa]

Currently we support Combo buttons, like SELECT+START => MENU, and long press SELECT => MENU, long press START => MENU. plus other combinations to do things like exit game.

if your controller has a dedicated HOME button, on iOS 14+, you can use the HOME button to get the MAME4iOS menu.

we should not do any combo buttons (for SELECT or START) in this case, just let MAME see the button.

[Alexck002]

Thanks Toddla for doing this PR

[ToddLa]

bigest change is to the HELP and README, just explaining how controllers work.