Add README with details about the status of support for this app

Bug: b/375598322
Change-Id: Ifb35d0b345a2650dd9beb8c333796ad3dba3def6

README

diff --git a/README b/README
new file mode 100644
index 0000000..ff1d3d0
--- /dev/null
+++ b/README
@@ -0,0 +1,3 @@
+This app is not actively supported and the source is only available as a
+reference. This project will be removed from the source manifest sometime in
+the future.
\ No newline at end of file