Add a note why the 'sed' hack is nessicary.

Originally committed to SVN as r2899.

aegisub/scripts/unix-gen-xpm.awk

@@ -11,6 +11,8 @@ all: bmp2xpm \n \
 CONVERT ?= %s \n \
 ", BIN_CONVERT)
 
+# Yes, the 'sed' hack is annoying, if anyone wants to know why it's nessicary
+# I direct them to coders/xpm.c:734 in ImageMagick 6.5.1
   for (v in image) {
     printf("%s_xpm.xpm: %s\n", v, image[v])
     printf("	$(CONVERT) -transparent \"#c0c0c0\" %s xpm:- \\\