This is also in the rules
I believe the select and start buttons were mostly used for menu navigation and not gameplay as often, so A + B buttons and d-pad are the main controls.
Common keyboard setups are:
D-pad: arrow keys or WASD
A/B: X/C* or J/K
Select: SHIFT
Start: ENTER
*EDIT: I am told that X/C are a better choice than Z/X as they work well for both azerty and qwerty keyboard users.