[FEATURE] Document pin pairs for ATmegaX boards

[nseidle]

Incredible library. Thanks!

I can't find any docs describing the required pins (if any) for the ATMega2560.

Are the complementary pin pairs for the ATmega2560 4&13, 11&12, and 9&10? I found them only in the source here.

I recommend adding a 'required pins' column to the Arduino docs page.