Small additions to the BCD lessons

@AntonioND :

I would add a paragraph mentioning that you can use this with sub, sbc, add and adc, not just with addition. And another one on how to chain multiple addtions: add+daa followed by as many adc+daa as required, and similar for subtractions.

See https://github.com/gbdev/gb-asm-tutorial/pull/115