Add README am: 3aeba4cab6 am: 591bf2432c am: f9cd093ca0 am: ec2af4c798 am: 87a4983a94 am: 022804f5da

Original change: https://android-review.googlesource.com/c/platform/packages/apps/Messaging/+/2614297

Change-Id: I50e9a6038785ea53e80277d01360b4a311b20f67
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
diff --git a/README b/README
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--- /dev/null
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+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.
+