This file is a reminder of what each color in the image's palette corresponds to.
It's not used by the program, but without it, I'd forget which is which.
Note that in the actual PNG file, the color 0 actually codes the transparency.
GIMP ignores it and starts the palette indexing on the next color, though, at 0.

0 : coat
1 : coat outline
2 : mane - primary color
3 : mane - secondary color (same as primary if the mane is only one color)
4 : mane outline
5 : pupils
6 : pupils - smaller part
7 : eyeliner (same color as coat if no eyeliner)
8 : cutie mark - primary color
9 : cutie mark - secondary color
10 : eyelashes (pretty much always black)
11 : eyes (usually white)
12 - 13 - 14 : supplementary colors (usually for the mane, but sometimes for other stuff - same color as mane if not needed)
15 : pupils (usually black)
16 : teeth (white)
