Fixed a glitch in contrib/pngminim/preader/README.

contrib/pngminim/preader/README

 This demonstrates the use of PNG_USER_CONFIG and pngusr.h
 
 To build a minimal read-only progressive decoder with X display, run
-    gather.sh  # to collect needed files from pngminus, libpng, and zlib
+gather.sh to collect needed files from pngminus, libpng, and zlib
+
+Edit makefile.std if required, to find your X library and include files,
+then
+    
     make -f makefile.std