The original ROM was modified to include a scene intro and to bypass the copy protection. The header was also modified in an effort make the title easier to see when selecting the game from a copier.
"Sonic the Hedgehog 2 (CENSOR prototype).md" is a modification that removes the intro and restores the ROM back to the way it was meant to be.

Source: http://hiddenpalace.org/Sonic_the_Hedgehog_2_(CENSOR_prototype)