DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/README.version b/README.version
new file mode 100644
index 0000000..58890dd
--- /dev/null
+++ b/README.version
@@ -0,0 +1,3 @@
+URL: http://prdownloads.sourceforge.net/libpng/libpng-1.6.20.tar.gz?download
+Version: 1.6.20
+BugComponent: 87896