[pngzop] Initialize "pngzop" branch, for investigating the "zopfli"

compression method for compressing PNG files.
diff --git a/README_master.txt b/README_master.txt
deleted file mode 100644
index b236c0f..0000000
--- a/README_master.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-
-/*
-  This is the master branch of the "pmt" tree.
-  Individual projects are in separate branches,
-  e.g., pngcrush is in the "pngcrush" branch,
-        pngmeta is in the "pngmeta" branch, and
-        pnguri is in the "pnguri" branch.
-*/
diff --git a/README_pngzop.txt b/README_pngzop.txt
new file mode 100644
index 0000000..e533b39
--- /dev/null
+++ b/README_pngzop.txt
@@ -0,0 +1,6 @@
+
+/*
+  This is the pngzop branch of the "pmt" tree.
+  Code for supporting the "zopfli" compression method
+  in PNG files will appear here.
+*/