Add a note about where to look for information on the mkhowto program;

recent confusion would have been avoided had the instructions been
followed.

doc/ZConfig/Makefile

 # Rules to convert the documentation to a single PDF file.
+#
+# See the README.txt file for information on the mkhowto program used
+# to generate the PDF and PostScript versions of the documentation.
 
 .PHONY:	default all pdf ps