Add comment about outline_size.

src/dft/dft.c

@@ -45,6 +45,7 @@ static bool initialize_freetype()
     return true;
 }
 
+/* outline_size is in 64ths of a pixel. */
 dft_glyph_t * dft_glyph_new(dft_font_t * dft_font, uint32_t char_code, int32_t outline_size)
 {
     FT_Glyph glyph;