Add RETIRED.txt to top level dir.
diff --git a/RETIRED.txt b/RETIRED.txt
new file mode 100644
index 0000000..a9a7fda
--- /dev/null
+++ b/RETIRED.txt
@@ -0,0 +1,3 @@
+This podling has been retired, please see:
+
+http://incubator.apache.org/projects/index.html#corinthia