[pngzop] Added commentary and license info to the new pngzop script.

pngzop

@@ -1,6 +1,17 @@
 #!/bin/sh
 
-# pngzop file.png > file_pz.png 
+# pngzop file.png > file_pz.idat
+
+# For use in the pngzop project
+# Copyright 2013 by Glenn Randers-Pehrson
+# Released under the pngcrush license (equivalent to the libpng license)
+
+# From a PNG file, finds the smallest resulting compressed IDAT
+# data from zopfli compression of filter types 0 through 4 and
+# "adaptive" (f5) filtering.
+
+# To do: replace the IDAT chunks in the source PNG file with the
+# result.
 
 for f in 0 1 2 3 4 5
 do