Added a readme file

Signed-off-by: santorac <[email protected]>

package-system/libpng/readme.txt

@@ -0,0 +1,2 @@
+Before building libpng, you must build the zlib package using build_package.py. For example:
+d:\o3de-repos\o3de\python\python ..\3p-package-scripts\o3de_package_scripts\build_package.py --search_path . zlib-1.2.11-rev1-windows